I need to jump start a 2008 Ford Focus and I am using my wife's 2006 S60 Volvo. This Volvo has the battery in the trunk and the owner's manual says not to use that for jump starts. Instead, it recommends using the engine compartment. Unfortunately, there's nothing more explained in the manual. Where is the positive and negative terminals under the Volvo hood?
You can use the trunk. The reason they say not to use it is for your safety so you don't shock yourself. If you're capable of hooking up jumper cables and have decent access to the battery it's perfectly acceptable. Many Mazdas, Chryslers, BMW, and Mercedes all say to use the engine bay because the batteries are under the seats or in the trunk; It's just so you don't hurt yourself.
Underneath the hood of your Volvo there will be a red cap with a bolt near the fuse box. This is your red or live wire. Generally when jumping a car, you want to have the black or Negative end connected to the engine to supply direct ground to your starter. This should work for your car.
The manual is half-wrong:
When acting as a power source to jump someone else's car, clamp the jumper cables directly on the battery terminals (in the trunk in your case). This provides the lowest impedance path to the other car's starter.
On the car you're jumping, clamp the jumper cables as close to the starter as possible. If the battery is under the hood, clamp them on the battery terminals. If the battery is in the trunk, clamp them on the jump-start terminals under the hood.
The negative terminal is usually just the lifting hook on the body of the engine, not a "nice" terminal somewhere else.
The location of the positive terminal under the hood is outlined in the pictures below. It can be either under a black flap:
or under a red cover (missing in this picture):