Mapping for HL1:Source

I don't have the original HL. I've only got the Source version.
How is it possible to map for it and HL:DM?

Um. Do you mean, "I own Half-Life 1 through steam" or "I only own Half-Life: Source"?

In the case of the first one: get Hammer 3.X working with Steam.

In the case of the second (you should use the half-life 2 editing forum, since it's a Source game), run the SourceSDK from steam and choose "Half-Life Source" as your game, or "Half-Life Deathmatch Source". Or whatever HL2 game you want to edit, then run the appropriate program (Hammer, Face Poser ... whatever).

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Blame it on Microsoft, God does.

It doesn't come up under my Current Game box..
And the .fgd file is nowhere to be found.

And I wasn't sure where I should post it.

EDIT: I moved this to the HL2 Editing forum.