The lock/unlock button detached from my Boxster's fob too. When I disassembled the fob, I found that the small metal cage that holds the button in place had deformed. After carefully bending the cage back into shape I was able to reattach the button, and it's worked just fine for me for several years.
If this fix doesn't work for you, it may be worthwhile to try to solder in a new button. That's what I had originally intended to do, and it really doesn't look like it would be that difficult. Of course, if you aren't able to fix your current fob you can always get a new one from your Porsche dealer.
Good luck getting your fob to work and do let us know if you are able to fix it.
More on my fob project here :
https://rx19jones.wixsite.com/projectboxster/projectboxster5