I had the most difficult time with Verizon Access Manager because of this issue. Evidently it doesn’t uninstall cleanly. Another place to look if you get the “A newer version of this software already exists on this volume.”
If you can’t find this folder, in Finder press Cmd+Shift+G and type in the path. Find the publisher, in the case of Verizon Access Manager it’s SmithMicro, and delete all the application files.