You may want to surf google for the answer to the question "street atlas no cd". It will likely involve a registry hack or multiple hacks.
I have both Garmin Mapsource US Road & Rec and Mickey$oft's Streets & Trips 2002 on my 510. I just have to twiddle the GPS's settings between Garmin and NMEA modes for my eTrex. It may not matter since got it to install, but I had to first copy the Mapsource CD contents onto the 510's drive, then tweak the registry to tell it where to find the image files. For the S&T2002 I had to do an admin install on a box that had a real CD drive, then install to the 510 via my LAN.
Hope this helps. I had to figure it out on my own and realize maybe others would benefit from my struggles.