The "Search" functions are not perfect.
To find a particular surname category out of the thousands, browsing though the whole of Category:Surnames is not appealing unless your target name comes early in the "A's". We are planning a work-around for this. Until it is working, one option is a simple search using the surname; but if that produces a very long list of pages try this:
http://genealogy.wikia.com/index.php?title=Special%3APrefixindex&from=Z&namespace=14, then you should see a box with "Display pages with prefix:"; if (as is likely) you've seen enough "Z" categories, replace the Z by whatever initial letter or group of letters you like and hit Enter; if the resulting list is too long for comfort, add another letter to the group in the box and try again. (The same trick works for other namespaces - just change the final "14" to another number. We may repeat this hint in other places relating to other namespaces that produce long hitlists.)