HP Photosmart C6100 and wi-fi printing

I have a wireless-enabled HP Photosmart C6100. I was successfully able to set it up using the instructions in the manual and the CD software, which says to create a computer-to-computer (ad hoc) network. I then go to the printer, tell it to see that network, and enter the appropriate WEP password. I then go back to my MBP and tell the Airport to connect to that C2C/ad-hoc network, which it does, after I enter the WEP password.

However, when I tell the MBP to connect to that C2C/ad-hoc network, I can't connect to the Internet, so if I want to print a page on the internet, I can't!

My internet here is PPPoE-based, so a username and password is required to connect. I have that username and password saved in my keychain, and set according to a particular location, but the C2C/ad-hoc network won't connect unless I change the location to Automatic.

Is there any way for me to stay on my wireless connection (via a router, which the printer can also see) and print? I've tried adding the IP address to the Add Printer utility, but that doesn't work, either.

Help!