You'll need to take the Frontline assets and put them in
"Steam\steamapps\common\Team Fortress 2\tf\custom\custom_models_and_textures"
(the custom_models_and_textures is a folder that you must create, as it is not already there, it doesn't have to be called that, but name it something relevant so you know where to find it)
Then load up Hammer and they should be there.