Install:
Admin -> Packages -> Download Packages -> Simple Machines Third-party Mod Site, click on browse.
Select SMFShop from the list. Chose download.
Err, then it's back to browse packages i think, and you select install next to SMFShop. After that it guides you through installing, asking you for ftp details, and basically clicking Next.
Do you need more help for actually configuring the shop? ie, adding items etc?