Q: Help Networking Mac w/USB WLAN Client Utility Adapter

Need help connecting Macs.

I have 3 Macs; an old G5 w/ 10.4.11, a Mac Book w/10.6.8, and a new iMac w/10.8.2. I also have an Airport Express Base Station. I am trying to get them all networked to share files. The thing is that the G5 does not have built in Airport. I use a Realtek USB WLAN Client Utility Adapter on that machine. Im thinking that bc of that I am having inconsistent results when trying to connect to this machine.

I have Sharing set up on all machines. Changed permissions to Read & Write for all. The G5 can connect to the laptop and iMac with no problem, but trying to connect from the Mac Book or the iMac to the G5 is problematic. The Mac Book (10.6.8) has more success, but I can never get the iMac to connect to the G5 unless I first mount the iMac on the G5, then if I go back to the iMac and try to connect to the G5 it will find it. Is there anything I can change in the settings that will help w/this or is it hit or miss bc of the adapter? Thanks for any help with this question.