There have been some really great work arounds posted for notating multiple measure rests, but I'd just like to add this one which has been working fine for me for quite a while now. Here's what you do to enter a multi measure rest:
1. With the cursor in the bar your want to notate as a multi bar rest enter text mode, (x), select 'Page text Times Roman ', type 6 spaces and then the number of bars rest you require. Go to the 'expression placement' tab and select staff position 4, uncheck 'preserve width'. Click OK.
2. Enter text mode again and type the pipe symbol, |, then 5 equal signs if the number of bars rest is a single digit, or 6 equal signs if the number is two digitis, =====, then type the pipe symbol again. Select staff position 0, check preserve width and click OK.
3 Enter text mode again, type 3 spaces and click OK.
This will give you a good looking multi measure rest, but here's the secret.....What you do now is spend a while making up a score comprising only of multi measure rests starting with 2 and going up to, well, whatever you like, mine goes up to a 20 measure rest. Then, before you start notating you load up your multi measure rest score first and then the score you're working on on top of it. Then when you come to a multi measure rest on the score you're working on just bring up the measure rest score and do a copy and paste on the meaure rest you want.
Hope this is helpful.