Well, the way I see it, you have a few choices if you really like the software and want to continue using it.
1. Don't bother getting a license key
and just put up with the nag messages. (it won't stop working just because you don't have a key)
2. Get a free 6 month license key
by registering on the forum and then generating one (you are halfway there, since you have already signed up...now you just need to go get that key)
3. Make a donation
to show the developer some appreciation for all the hard work he has put into that application. In other words, just like buying shareware, except you get to decide how much it will cost. The bonus is that the license won't ever expire and you can use it for ALL the software on the site that uses a key. It's a fantastic "buy one and get everything else free" deal, because you get to set the price of that one. Not too many software companies do incredible things like that!
4. Contact mouser privately
, and tell him you are either too broke to donate or genuinely opposed to supporting the development of the software that you use, and he will likely just give you a permanent key.
Now what other software company offers you this many choices and is so willing to bend over backwards to make you happy?