I hope you have already gotten the answer to this, but if not, do the following:
I use a USR 56k faxmodem, unsure if other modems will work the same
Dip switches 3,7,8 in the dwn position...all others up,
Using an RS232 cable with an RJ 45 connector on one end and a 25 pin connector on the other (I use an RJ45 RS232 with a RJ45 to 25 pin connector to the modem) connect the modem to the AUX port of the router. Then use these commands from the enable prompt
line aux 0
modem InOut
modem autoconfigure type usr_sportster
transport input all
stopbits 1
speed 115200
flowcontrol hardware
Then use hyperterm or similar to dial in. Good Luck
Cisco has a list of cables on their website in case you do not have the correct RS232 and adapters.