In UserCP, simply click on categories, click on the category you want to change and set the new permission, but remeber ... permissions are inherit to sub/child categories.
You have to change the "main" categorie 1st

I've tried doing this and it still doesn't work.

My main category is set up thus:

Registered Users
Administrators

Yet when I try to set the permissions on the sub galleries I do not get the option to select these - for example, in the sub gallery "UK Magazine Ads + Trade Related Scans" all I can select are:

Buddies
Friends
Administrators

So it seems the permissions are inconsistent.

Support Xcache?

Sorry, you've lost me - what does that mean?

I can't replicate this error.
I've created a category called: RA and used "registered users" & "administrators"
When I now create a new category and choose as main "RA", it shows me:
"registered users"
"administrators"

What would you suggest?

I am also unable to create any new categories as I just get the "Category Structure" tree when I click on Admin -> Categories.

Can I have an test-Account for the UMG (incl. Adminrights for UMG) ?

Or ... what happend, when you try to edit the Child-Category (switch the main-category to another child-category and then back to the main-category) ?

I've deleted all the galleries and started from scratch. I'm creating public galleries, and I've got to the point again where I am unable to create any new categories. Clicking on admin -> categories just lists a Category Structure. How do I navigate to the Add Category page?

There is no "real" mistake and you didn't do something wrong.
Take a look at my PM, to see what I've done.

Cheers

I ran was able to convert from photopost. I get all of my categories and thumbnails but when I click on the thumbnails they do not go to full size, I just get a blank page.

Any suggestions?

EDIT:

http://www.howieforums.com/forum/umg.php?c=16

If you go to that link the most recent upload, the screenshot you can see it works. If you click on my other thumbnails you will see what my problem is.