I just checked a 1.8.6, the categorization list is in tiki-editpage ... so it is very strange. Check another theme to see if it was deleted.
Whatever, to insert a categorization in an object edit page it is easy:
In the php you need to do an
This will set up all the variables.
And in the associated template, you have to insert
missing page for plugin INCLUDE
If you don't have these files in your version you can copy them from a 1.8.6