If you are using a the most recent Gramps 5.2 code (after beta1), from the AIO or from Github, you may have noticed that attempting to load Isotammi addons via the Projects tab of the addon manager doesn’t work; it causes an exception.
I’ve patched up a build file to update the listings in my personal Github account area – so this should work now.
Add the following to your Addon Manager/Projects tab:
https://raw.githubusercontent.com/prculley/isotammi-addons/master/addons/gramps52