Do you experience problems during downloading?
Check your connection in Status and Interface settings window.
Use Test Server in Config > Servers.
We will probably ask you to do a test using only basic settings.
Do you experience problems during repair or unpacking?
Enable +Debug logging in the Status and Interface settings window and share the relevant parts of the log here using [ code ] sections.
For older Ubuntu versions, use the custom repository maintained by user jcfp. See this thread for details.
I installed a brand new Ubuntu 10.04 server and ran apt-get install sabnzbdplus and it installed 0.5.0. Am I missing something? The way the download page reads to me is that the extra repository isn't necessary anymore. Am I incorrect?
Yes, you're missing the last line of your quote: you do have an older Ubuntu version, haven't you? So if you want the recent SAB, use jcfp's PPA. Check out https://launchpad.net/~jcfp/+archive/ppa