I got my parts kit yesterday with a romanian bayonet.  I pressed the button in on it to release it after mounting it, and it is stuck now.  The button is stuck down.  Right now I have been soaking it for the last 24 hours in WD40 but it is still just as stuck as it was yesterday.  Any idea how to get this thing to unstick?  

Try spray with gunscrubber or brake cleaner as it may have cosmoline in it making the internals stick.  Poor boiling water over it, that will loosen any gunk in there, it will also heat up/expand the parts and may allow you to get the button working.  Use a different lubricant such as Kroil, remoil, liquid wrench, etc and see if the lube will get in there and loosen it up after cleaning.

This is off the barrel, right?

yes, it is off the barrel.  it is stuck so that I can mount it but it cannot lock onto the lug.

I bought one that was like this, with the button stuck down.  

My button was binding on the bakelite 'sleeve' built into the handle, the one that the button rides in.  I used a utility knife to carefully carve a bit of plastic off the 'sleeve', thereby giving the button more clearance.  Once I had removed some plastic, I wacked it a couple times and the button popped out and hasn't gotten stuck since.

take care,
Tec