Do I need a SCC connector add-on on a R55 smartcenter (running on a windows XP machine) to add a safe@office gateway to stablish a site to site VPN? Another words, I am trying to setup a site to site VPN between my SecurePlatform box (R55 with AI FW-1, VPN-1) and a safe@office 500. I would appreciate your help.

In case you would only need to create a Site-to-Site VPN, it is not required to have the connector add-on.
There are some important notes, though:
1. SmartCenter can not manage Safe@Office devices! if you would like to be able to manage it, you have to use a UTM-1 Edge device.
1. When installing the add-on, you will be able to manage all the aspects of the edge, and manage its VPN just by joining it to the community.

HTH,
/Shachar