More helpful response:
-Addons are updated individually by their authors (or sometimes fan updates are released) so new versions come out when they finish the newer version and get it uploaded.
-If you don't like the constant pop-ups I'd recommend grabbing BugSack. While it won't get RID of your errors, it will make them easier to read/copy/paste as bug reports to the authors.
-As a general practice (on this site at least), stick to only downloading addons with a 2.4/Updated!/Star next to the name. These are addons that are updated to:
A) Work (generally) in 2.4
B) Have been updated recently (anything older than a year isn't worth downloading, as it's most likely (MOST likely) going to have issues
C) Is popular and downloaded by many people
-Most pre-made UI's are comprised of the most updated addons so really contrary to what noble said, building your own might make more problems if you can't find exactly the right addons
I'd personally suggest getting WoWMatrix (3rd party program) as it scans a couple of sites to find updates for your addons.