I bought Donor rank about 13 hours ago and it hasn't registered that on the server.
I know my username is correct because it showed my skin in the recent payments on the website.
Should it be taking this long to come through or is this an error?

 

This is most probably an error.
http://skyblock.net/threads/report-here-if-you-are-missing-donation-perks-net.17143/ <-- If you are .net go here.
http://skyblock.net/threads/report-here-if-you-are-missing-donation-perks-org.17144/ <-- If you are .org go here.

Hope I helped
-Jaff