WEP is very insecure... and I would not use WEP for my wireless network security. All contemporary equipment (I think since 2004) supports WPA2. I would ditch the old printer before I lowered my security to WEP. Alternatively, I would attach it via wire to your router, or via USB to one computer and then share it.
use the airport utility to activate wep on your network and/or assign a wep keyphrase. if you set up your network then you should know the keyphrase you created.