Buzz, I had the same thing-- are you using the desktop app? There is a drop down box there, it is blank so you don't notice it, and you have to select it and then click on your vacation... it will THEN let you hit "apply". I ended up going back a year reading all the sched papers sent out to figure it out, could NOT figure out why it wouldn't let me enter it!
Good luck.

NOW what you have to think of is are you going to check the "Else Start Next" on it? If you CAN'T slide your vacation, you may want to have another bid group to ask for trips with vacation as is... if you CAN slide it, you may want different things. At least, in my case, a 2 day slide would make a big difference in what I ask for as it puts me close to Thanksgiving and the delicate dance there to get the days off.

see PBS notepad 12-02 "New Features for May 12" for info. You also have to make the range NOT INCLUDE "0", which is the same day. So you can slide 1-3, or exactly 3, but NOT -1 (day prior) to 3. And you can't slide over any other existing event or any portion into another month.