On the 515 you shouldn't need anything special other than a nat/global which you should already have. On the 501, because traffic will be coming from outside to inside, you will need a static and an access-list. The access-list should just permit port 9100 (TCP or UDP?) and users will print to the global address of the printer that is defined in the static command.
In short, it shouldn't be any different to allowing any other IP traffic from a lower security interface (outside) to a higher security interface (inside).
