If your daughter is currently logged into her account on the Mac's iTunes then you log her out via the Store > Sign Out menu option, and you should then get a Store > Sign In menu option so that you can sign in with your account. But if she is using iTunes Match, automatic downloads or she's redownloaded past purchases then it might be tied to her account - and if not and you redownload any of your purchases on it you may tie the Mac to your account for 90 days : http://support.apple.com/kb/HT4627
You can authorise your account on it via the Store > Authorise This Computer menu option, but an iPad and iPhone are designed to only be synced to one computer at a time, so if you do connect them to that computer you risk replacing all of your current content on them with what is on that computer (syncing with a new computer)
If you authorise your account and on the Devices tab in iTunes > Preferences (command-comma) tick the 'Prevent iPods, iPhones, and iPads from syncing automatically' tickbox you might be able to copy your iTunes purchased music over from your iPad and/or iPhone via File > Devices > Transfer Purchases
thank you, I think that makes sense as to why I cannot see the option in manage my accounts to add this imac computer, it just shows the iphone and ipad devices. I will probably not bother then as dont want to dissacociate as travel with ipad etc freqently. I will just content myself with having no music on imac whilst working and use ipad instead. Not the end of the world, although apple spoils us to expect a lot? lol
the imac is on loan whilst daughter away for some months but dont want to make any risky changes and have my own login to keep my stuff seperate but will err on side of caution as still use my old lap top PC when away so dont want to scupper that eithe!
thanks for your reply. King Pengu,