The following is an excerpt from page 18 of the Warhammer Annual 2002 release with an expanded explanation by Gav Thorpe regarding deploying war machines.

In most scenarios, all the war machines in your army must be deployed at one time. They don't have to be deployed near each other, and there are no battery rules. Characters can be deployed with them when your characters are put on the table (such as Dwarf or Empire Engineers), not when the war machines are set up.

War machines (or anyone else for that matter) cannot be deployed in impassable terrain. Now, this is where arguments start, as players think that different types of scenery would be impassable to war machines - we've probably all heard someone say, "How did that cannon get on top of that tower then? Air-lifted by Griffon was it?" or similar. As with all terrain, there are no hard and fast rules to apply because people's collections can vary so much. However, it is always a good idea to go over the different scenery pieces on the battlefield before the game starts and say what type is what. Whilst doing this, it is worthwhile agreeing any areas where you feel war machines cannot deploy, thus saving debates once half your army is down and your whole deployment is based around setting up your Volley Gun in those rocks.