Allright, this one should be fairly easy - how are you trying to let the partner companies in? Are you setting up an IPSEC tunnel between you and the others, are you allowing users from the companies to create a PPTP tunnel through, or are you just simply trying to open up specific ports on the firewall?

The absolute simplest way to do this is to create user accounts for the companies, set them up as PPTP users, and create a passthrough rule something similar to the following: PPTP User A, once authenticated to the Firebox is allowed complete access to the IP address of your file server. On their end, they simply set up a new VPN connection, use the external address of your Firebox and input the username/password combination that you assign to them and they should have access...