The iTune/Apple Store password is the one you set up to make purchases online there. On the other hand, the Password that Office for Mac is asking for when you attampt to install it on your computer from the disk is your computer's System Password that you set up when you originally purchased and did the initial setup of your iMac. You will have to look up or remember what exact Password you used when you first set up your new iMac. They are two different passwords.
Hope this helps