For some reason my Outlook address book won't let me add anything to it. In my folder list it's showing my contacts folder to be full as usual but it won't show up in my address book when I click the icon on the toolbar or if I want to add an address to an email via the 'To' button.

Does this make any sense to anyone?

In Contacts\Properties, on the tab "Outlook Address Book" is the check box ticked for "Show this folder as an e-mail address book"?

Wasn't sure but I just checked and yes it is. Weird eh?

I did a google on "outlook address book cannot add" and came up with this:-
http://www.york.ac.uk/services/cserv/help/mail/Outlook/start/lostaddressbook.htm

I cannot see any contacts in my Outlook Address book

If you can see your Contact information in your Contacts folder but are unable to see them when selecting Tools | Address Book... follow the instructions below:

* Open Outlook and choose Tools | Email Accounts...
* Click on Add a new directory or address book, then Next >.
* Select Additional Address Books then click Next >.
* Choose Outlook Address Book from the list of additional address book types, then click Next >.
* You will be prompted to close Outlook and reopen it for the changes to take effect.
* Open Outlook and right click on the Contacts folder on your Outlook Today shortcuts.
* Choose Properties.
* Click on the Outlook Address Book tab.
* Check the Show this folder as an e-mail Address Book box.
* Click OK and check you can see your contacts in the address book by choosing Tools | Address Book...

There were lots of google hits so you might want to trawl through a few and see if anything else matches your problem.