Assuming you have Steam installed and Sven Co-op is fully updated and working, this is what you need to do to find out what your Steam ID is:

1:  Load Steam

2:  Load Sven Co-op

3:  Join an online game (or if Starting a New Game, ensure the "LAN Game" checkbox is disabled in the Game tab).

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4:  Once you are in-game, bring up the console by pressing the tilde key. Type status, then press Enter.

5:  Your STEAM ID should be displayed beside your name (STEAM_0:0:1337 in the example above).

Write it down, or copy and paste it out. You could take a screenshot if you wanted to be extra careful.

Alternately, the website https://steamid.io/ may be of use.

Finally, send me an e-mail with the subject "STEAM ID". Include your full Steam ID (e.g. STEAM_?:?:????? ), the e-mail address and name you used when donating, and any other relevant information. I'll send back a confirmation e-mail as soon as I can manage. If you don't see a reply after ~48 hours, please check your spam folder.