I can't log into my SMC Barricade G router. Its IP address is supposed to be 192.68.2.1, but I can't get to it from my browser (per the manual) or ping it from the command line. Yet it is working just fine.

I have
3 computers -> SMC Wireless router ->Actiontec DLC modem ->Qwest DSL

The SMC is conected to the Actiontec by ethernet cable. I can log in from any computer to the Actiontec (192.168.0.1) and get its admin screen, but I can't get into the SMC at all. Is it possible that someone has changed its address (I ran unsecured for some time)? How can I find out its address, and also log into it?

The gateway defined in your TCP/IP settings is either the router or the modem.
Other than that all you can do is scan your network for IP addresses or just reset the router to factory settings.

BTW it looks like your modem is a router too.. Why do you keep that SMC thing around at all?

Use the little green Reset-button on the front of the router. Hold it in for about 10 seconds. That should reset your router to factory installation.
And the standard login is 192.168.2.1

Two reasons: (a) the SMC was already in place, used for file and printer sharing before I got DSL, and (b) it has a built-in print server I was hoping to use. The the DSL installer would not help me reconfigure my computers for the Actiontech. HE said "We don't do networs, just provide DSL". He just disconnected the SMC and said I don't need it. But next day I found out that file and printer sharing didn't work anymore, and fooling around for two hours with the Qwest help desk, the solution and plug the Actiontech directly into the SMC and turn off the Actiontech's transmitter. But now I can't get into the SMC.

I will try the next suggestion and reset the router. But I would like to know how to scan for IP addresses.

Any portscanner will scan a range of IPs and you can tell it to ping the target. Or just scan for port 80 which the router has to have open for the management page.