OK, it would seem that your imac is preventing you from connecting. I'm a little out of my depth when it comes to Mac OS's, typically I would recommend adding your username and password that you are using on the Win 7 machine to the machine that you are having issues with. If you can do that then perhaps it would help, otherwise examine the printer permissions and see how it is shared on the iMac. Are there any shared folders on the iMac and can you access them from the Win7 machine? If you can see it, you should be able to access it once you overcome the permissions issue.
EDIT: Is there any type of firewall settings or file and printer sharing setting that still need to be adjusted on the MAC?