I didn't write down my bitcoin wallet code (that long alphanumeric number that was assigned to me) when I installed the software. How do I find it?
If you are using the Bitcoin-Qt client, then the address you might remember seeing is just the first address. You can see that address, and any others that you've added by clicking "Receive". [Edit: writing this from memory, without a Bitcoin-Qt client to know if I have the right name for the tab.]
If you are using a different client, the steps will vary.