Taxonomy picker control disabled:
You might need to go to the library settings and make sure it is enabled for keywords. Try the Enterprise Metadata and Keywords link. Go to the column [wiki categories] and then select the term set that you would like to use? It remains grayed out until you select some term set to start with :).
If it’s a site column: Goto site settings > site columns > edit the column(managed metadata column)> select the appropriate metadata field> OK