Hello, my name isXXXXX will do what I can to get this straightened out for you.
So you have had the Lexmark and the iMac for just a week?
One problem is that Lexmark drivers are included with the operating system and do not need installation. That may be part of the problem but is easily remedied.
Who are you sharing the printer with?
I don't think we're talking about the same type of share. What type of router do you have?
According to the product sheet it is wireless enabled, which means it can join your wireless network.
According to the Lexmark user manual for that printer the necessary steps to set it up are on the CD that came with it. I do not have those instructions and they are not really provided on the website.
The best way to add any printer is to do so through the System Preferences/Print & Fax.