Well, first, go to your forum's admin section. Then, choose Package Manager on the left-hand side (I believe this is called 'Packages' in the SMF 1.1 RC2 release). Once you're there, choose 'Download New Packages'. Choose where you downloaded the SMFShop package to, and click OK.
If you need any further help, please feel free to ask.
P.S Glad to hear you switched over from phpBB! Trust me, you won't regret it. Most of the things that require MODs in phpBB (like attachments, subforums, etc, etc, etc) are in-built to SMF. Also, it has heaps of other useful features.