You have to put hammer++ to your hl2 game folder. To dowload a hl2 hammer ++ do a gmod hammer++ tutorial but change everything as hl2 (I thnikkkkkk)

In order to have access to all 3 games' assets, the easiest way would be to just map for HL2:Ep2. That is pretty much the standard for HL2 mods, anyway. That means putting Hammer++ in the bin folder of Ep2, not HL2 itself. If you really need to use HL2 for some reason, you'll have to manually copy the Episodes' assets over to its files.