Some extra info I discovered in case anyone else stumbles across this question later:
In Sharepoint Online (2010), you get to the Metadata editor by going to Site Settings > Site Administration > Term Store Management. It's also available via the admin site under 'Term Store' in the main menu.
In order to edit the terms, you need to give yourself Admin rights for it (in the right hand pane of the term store). This step confused be a bit since I was already logged in with an administrator account.
To get your newly created tags to appear when you edit properties on a page, you need to add the columns to the content type 'document'. You do this through Site Settings > Galleries > Site Content Types and then select the 'document' item under the Document Content Types heading.