You'll be able to use the PM service once you are an "established" member. This restriction was necessary because the PM's were being used inappropriately in a number of ways. For example, many people were using PM's to ask specific people Tech questions which meant that information which could have been useful to the entire Community was instead kept out of the database. And often, it was new members initiating these private exchanges because they were "shy" about posting publicly on the Board. So, disabling PM's for new members is our way of encouraging public posting to the Board. There are other reasons as well, but this is by far the most important. It is not a perfect solution - obviously sometimes a new member may have legitamate use for PM's. We are looking for other ways of ensuring that PM's are used appropriately - perhaps when the release version of our vBulletin program is available there will be some better options to control their use. In the meantime, the restriction is necessary for the good of the Board as a whole.
If you have a specific need to communicate with a member privately you can email Sal to see if he can hook you up.