Not sure if Source has the same rule but in the older HL (CS 1.6 particularly), you would have to copy the map files to $steam_dir/cstrike/maps and then add the map name without the .bsp extension to $steam_dir/cstrike/mapcycle.txt (like de_dust for de_dust.bsp). Change cstrike to hl2dm and see if it works.
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