From the Add or Edit Subscriber page, if the category you would like to choose does not appear in the list of categories, the category is not Active. 

To make the category Active, go to Lists and choose the option for Categories. On the following page, you’ll see the list of your categories and in the Active column next to them it will say either Yes or No. If the category you wish to use says No then it is not active and you need to activate it. 

Click the Edit link next to the category you would like to change and on the following page, tick the box for Active. Click Save Changes.

You will now see this category in the list of categories on your Edit/Add Subscriber page.