To elaborate on what kaidus said and to repost:
sourceforts should be in:
C:\Program Files\Steam\steamapps\SourceMods\sourceforts
Within that directory should be the
gameinfo.txt and
changelog.txt as well as source-ish directories such as
bin,
cache,
cfg and others.
If these files and directories are in \steamapps\SourceMods\sourceforts\sourceforts you are doing it wrong.
If those files and directories are in \steamapps\SourceMods\ you are also doing it wrong.
If you do not own HalfLife2 Deathmatch you cannot play this mod until you do.
Please note: HL2DM= Half Life 2 Deathmatch
HL2DM≠HL2, HL2DM≠HL2e1, HL2DM≠HL2e2, HL2DM≠CSS, HL2DM≠HL2 Lost Coast
Also note: HL2DM≠Half Life Deathmatch, HL2DM≠Half Life Deathmatch Source