Need some Host Gator help

[DonkeyKongSr]

I recently signed up with HostGator and I'm having trouble getting my second domain going. Here's the basics (with made up URLs)...

My account is under mymaindomain.com. That site works fine.
I have my2nddomain.com registered and pointed to HostGator over a week ago. I put it as an Addon Domain. Now, here's my problem.
I've tried FTPing to just about every folder possible and I can't seem to find the one that will make my site show up on www.my2nddomain.com. I get a really, really long page load at that URL before finally an error page shows up. www.mymaindomain.com/my2nddomain shows the site, but I can't get it to go under the 2nd domain.

Am I missing a step?

Which folder do I need to put my files in? I thought it was public_html/my2nddomain, but that isn't working.