do a forum search guys for unit visits. loadsa info on here its been talked to death....
In my recent experience, there has been NO problem getting a visit at a unit. Phone the unit you want to visit direct. Usually they will only accept a visit after you've passed the initial selection tests and before you go to interview. If they can, more than likely they will give you a date and time! I visited LTCC and LTN tower in the same week....
good luck
C