If you have GitExtensions application installed in the system, this plugin should be able to find executable file automatically and work properly.
If you are using portable version of GitExtensions or plugin can't find the executable, you can provide path to the GitExtensions.exe file via IDE settings menu (File | Settings... | GitExtensions).
On Linux you can try to create simple shell script, for example
#!/bin/bash mono /home/user/GitExtensions/GitExtensions.exe $1 $2 $3
and provide path to your script instead of GitExtensions.exe file via IDE settings menu (File | Settings... | GitExtensions).