Well, let's check a few things:
1. Did you purchase it on your web browser or from within the Impulse client? (It should not matter, but I once had an issue when buying a game using my web browser instead of the Impulse client)
with a webbrowser.
2. When asked for your email address at checkout, did you use the email address to which your Impulse account is connected?
yes. also, if i use "lookup serial number" from within impulse, i get emailed the serial numbers of all my games, including TRINE!
3. In the Impulse My Games tab, click the View button and make sure that "Hide uninstalled games" is not checked
no, its not checked.
when buying the previous two games (braid and world of goo) the games did not show up in my games tab by themselves. i had to manually add them via their serial number using the "register product" button. i wanted to do the same with trine, but at the "register product" dialog you have to choose the game via a drop-down menu, and trine is (still) not listed there.
i guess ill contact customer support. but thanks for your help!