Well here's what I see when I open the free version of Malwarebytes:
http://i59.tinypic.com/123up39.jpg
Note that, at the top of the window, the grey bar shows 'Malwarebytes Anti-Malware (Free)'. It's that word 'Free' that confirms I've got the free edition. It also appears alongside 'License'. If you've got the same thing visible then, obviously, you've also got the free version (but, from what you've written, I still strongly suspect that you haven't).
If your screen is similar to mine, 'Fix Now' and then 'Scan Now' is all that should be needed. If not, uninstall and reinstall (watching out for the tick!).
>>>to be honest, I'm not an idiot
Neither am I, but I've still managed to accidentally install things (such as unwanted toolbars) in the past, simply through not spotting something something that I really should have done. We all do it!