Step I.) To begin, load Source SDK.
Step Ia.) Under Engine Version, change to Source Engine 2009.
Step II.) Click on Edit Game Configuration
Step III.) Under Current Game Configuration:, click on add.
Step IV.) A window will appear where you can input the name and directory of the game you wish to map with using Source 2009 SDK.
Step V.) For Name, enter the name of the game you will be loading into SDK. In this case, it will be Half-Life 2: Deathmatch. Other names are suitable.
Step VI.) For Directory, click browse... or manually input the directory of the installed game files. For deathmatch, it will be <install drive\directory>\Valve\Steam\Steamapps\<account name>\half-life 2 deathmatch\hl2mp
Step VIIa.) In completing these steps, you will need to close Source SDK and re-open it.
Step VIIb.) Once the Source SDK menu has reopened, select Source 2009 under Engine Version, and Current Game, select Half-Life 2: Deathmatch.
Step VIII.) Now load Hammer Editor, and from the dropdown menu, select Tools -> Options...
Step IX.) A Game Configurations window will appear, and make sure Half-Life 2 Deathmatch is selected as you Configuration.
Step X.) Below this you will see Game Data files: where there currently should only be one FGD file loaded - $SteamUserDir\sourcesdk\bin\orangebox\bin\base.fgd - You will need to change this!
Step XI.) Select $SteamUserDir\sourcesdk\bin\orangebox\bin\base.fgd and click Remove.
Step XII.) Once you have done this, click on add, and select hl2mp.fgd from your ..\sourcesdk\bin\orangebox\bin\ folder.
Step XIII.) After you completed the final step, it may be necessary to close and restart Hammer to load the new FGD file and hl2mp entities.
If you're planning to map for other Source-based games besides Half-Life 2: Deathmatch, follow the instructions as previously stated above. The exceptions are that you will need to load the FGD (Step XII) relative to the game you are mapping for, along with its name and directory (Step V)
Half-Life 2 - ..\sourcesdk\bin\orangebox\bin\halflife2.fgd
Counter-Strike: Source - ..\sourcesdk\orangebox\bin\cstrike.fgd
Team Fortress 2 - ..\sourcesdk\orangebox\bin\tf.fgd
Day of Defeat: Source - ..\sourcesdk\orangebox\bin\dod.fgd
Source Base 2009 - ..\sourcesdk\orangebox\bin\base.fgd