The cleanest method to both install and uninstall DM Extensions 5.1.x is to use the Deployment Package Wizard. With this tool you can create a silent unattended install which can be run from a network share. The format of the install string is \\servername\sharename\setup.exe dp="\\servername\sharename\deployment\packagename\packagename.ini"
One thing to add to the packagename.ini is SkipFinishDlg=Y under the [Hummingbird] section. This will suppress the Finished dialog from appearing.
To uninstall DM Extensions 5.1.x use the same Deployment Package Wizard, however click the 'Clear All' button in the wizard to deselect all components. The resulting ini file will then remove all DM Extensions features from the client pc. The same SkipFinishDlg=Y ini option should be added to this packagename.ini file too.
The Deployment Package Wizard should be run from your DM server: Start > Programs > Hummingbird > DM Extensions Server Setup.