In the 737/800 I have clicked on ACARS on the menu in the FMC and now find that when I press any key e.g CRZ or DESC I get the message "logoff acars first". How do I do this?

You go to MENU key and the you have a option at 6L LSK "Logoff" or at 6R LSK "HLD" this means that you don't logoff the ACARS but you putt it in standby mode. Use one of these two options and your able to select any page again you want.

Many thanks !All working now :-)Phil

Ok, your welcome, have a nice flight.