Really not sure what this falls under.
I’ve just gotten my first LulzBot (a Mini) and installed Cura 15.02.1 from the LulzBot website download package on a WIndows 8 64bit machine. I start Cura, and the “Control” button is greyed out – not clickable. There’s no error messages or status displays (I’ve never used Cura, so I’m not sure what I should be seeing). I can load and manipulate models, but not print.
My machine settings give me the option of COM3 or Auto. I’ve tried both, no dice either way. If I unplug the USB cable, COM3 (Rambo board serial port) vanishes from my Device Manager. I’ve tried both USB ports, direct connection, no luck there either. No hubs in the way. I doubt the USB ports are bad, anyway, since I use this computer to control my desktop CNC mill with no issues.
The only odd thing I can find is that my “Machines” list includes the Mini, and a PrintrBot Play. This computer was once used to control a Play briefly by a friend I loaned it to, but he didn’t have any issues with Cura. I did take the precaution of un-installing the older Cura install before I installed the LulzBot version, and I even installed to a different directory path, but for some reason the PrintrBot still shows up in the list of installed machines.
And, just to ice the cake, I’ve rebooted the computer and the Mini several times. No change.
So, my only two ideas currently are: 1. some weird software config issue, possibly caused by leftover config files from the PrintrBot, or 2. Some sort of odd connection issue. But I’ve gone about as far as I can with this – I’m open to suggestions on how I can narrow down just what the heck is going on here? I have gone through Google and the LZB forums looking for anyone encountering anything similar, but no luck.
It sounds like it sees the printer as you have a control button. If it does not see the printer it will usually be a save G-Code button. So it might be the model does not fit the bed size or is not print-able for some reason.
Well, I seem to have stumbled over the answer. Basically, the control button remains grey until I have a valid model on the build plate. Only then does the button un-grey (is that even a term?).
Assuming this is intended behavior and not simply a fluke of my machine, I have to say, that’s a darned odd design choice for a user interface. I wasn’t even trying to print (yet), just warm up the hot end so I could remove the stub of HIPs still left in the extruder from the factory octopus print, and load fresh filament. I was fixated enough on that that I didn’t even try loading a model until after I gave up on getting the printer to run, posted my original message, and then started to play with loading STLs for lack of anything better to do.
I will say that, once I got over this hump, the Mini printed its first few test prints like a champ – my first actual good out-of-the-box 3D printer experience, and I’m a legend among my acquaintances for having incredibly bad luck with 3DPs – my “lemon rate” for DOA units is so far above the statistical norm you probably wouldn’t believe me if I described it in full detail. I’m almost afraid to get my hopes up, but I have a guinea pig – that is, I have a friend who bought a Mini ~6 months ago and worked it like a rented mule since, with nothing but good things to say about its performance (and that includes a PEEK liner that went bad and required mailing the printer back for repairs).
Once you load a model, the control button stays greyed out?
Looks like you got it figured out. Yes, right now you cannot control the printer without loading a model. I agree that it is a bit confusing, hope to make this simpler in a future release.
Good to hear that your Mini is behaving itself 
Sorry to necro this thread, but it was the first result on Google so I figured I might help others.
the button is greyed out until a > valid > model is on the build plate
I was testing the max print size by numerically setting x, y, z dimensions to the very close to max (spec sheet says ~152mm cube) and Cura greyed out the control button - even though it fit on the virtual print bed and is advertised as capable of these dimensions. The ‘scale’ button has a submenu item for ‘max’, which resulted in a slightly smaller build dimension (148mm cubed).
For reference, I was using this STL:
If you want to print close to the MAX size you would need to go into Expert mode settings and turn off the skirt generation which takes 6MM of the max model size.