I believe there's an O ring inside the head where the priming button is. If you can't push the priming button down, the O ring may need lubricating with vaseline.
I'd start it empty, not full. Once the syphon starts, it'll fill.
One way to get it primed in an emergency is to detach the pipes from the filter using the quick release thingie (so water can't drain out the bottom of the pipes), then fill the inlet pipe with water (easier if you have one of the eheim installation kits on it, but possible with the normal green inlet by pulling the piping off the inlet, pouring water in using a small funnel and carefully reconnecting). With a little water in the inlet pipe, when you reconnect it to the filter it releases the quick-release, the water which was in the inlet falls down into the filter and it will suck more water into the inlet, starting a syphon.
With the installation kit, it's easy because it has a little cap you can screw off at the top of the inlet pipe which you can pour water down. Whichever way you do it, just make sure it's not connected to the filter. The idea is just to fill the little bit of pipe between the quick-release thing and where it connects to the inlet pipe. Works for me, at least.
Or, if it's clean, just suck on the outlet until you hear water falling down into the filer and begin filling it.
! If i use considerable force i can get the prime button to go down, but it won't come back up (which would actually do the priming). Is there any other way i can get this to prime?