Once you have downloaded the SGTLS Mapping SDK and installed it start the Source SDK application, set the engine version to "Orangebox" (also known as the Source Engine MP) and then open the Hammer editor.
This is due to a known issue with the Source SDK which nothing can be done about. A workaround you can use is to use Source Engine 2007 to do your mapping and then compile the map using the Source Engine MP (Note: You will need to create the STLS configuration for both engines)
Some basic models/prefabs are in the mapping SDK [ link ] other then that the only models/prefabs that are available are the ones released with the mods base file set. If you rummage through your SGTLS files folder sets you'll find lots of models/prefabs available to use which are not necessarily used in the mods official three maps.
A custom file [sgtls.fgd] for the Hammer editor containing the SGTLS mods entities made by a community mapper named Rakshu can be found on the community web site here. In addition, the huge list of the mods entities/props/objects was put together by Senseit and can be found on the developers web site here. There also in that thread is the weapons list and other general information which you might find helpful for mapping for SGTLS as well.