If you're seeing an empty or incomplete list of apps in the Installed tab of the Apps section of MacUpdate Desktop, you may have some bad scan data.

In order to correct this issue, first quit MacUpdate Desktop completely. 

Then relaunch MacUpdate Desktop while holding Command + Option (⌘+⌥). When MacUpdate Desktop has been launched, manually rescan for updates by selecting in the menu bar Apps > Check for Updates (⌘⇧U). This should regenerate MacUpdate Desktop’s scan data and resolve the issue.

If the above step fails to do the job, you will need to log out of the MacUpdate Desktop app and your account on macupdate.com, then clear out MacUpdate Desktop’s support files. In order to do this, delete the MacUpdate folder in the hidden Application Support folder by launching Finder, then selecting in the menu bar Go > Go To Folder… (⇧⌘G) and typing ~/Library/Application Support then deleting the MacUpdate folder. After doing this, download and reinstall the app, then restart your Mac and launch MacUpdate Desktop.