The osatch tutorials are awesome as far as signal placement. There's something to keep in mind. The AI drivers don't always follow prototype setups, so you might have to fudge things to work. Sometime thinking too hard on the subject doesn't help either.
If you're looking for the signals themselves, check under the Track placement, then trackside objects tab. There's a ton of different signals from all over the world. Choose the signal, click on the track with the + button, and the signal is placed. If it faces the wrong direction, choose the curled arrow to face the signal in the right direction.
John