I don't see MOPs when I boot up Houdini!
First of all, check the wiki for detailed install instructions: https://github.com/toadstorm/MOPS/wiki/Installation
Please consider using the package (JSON) method to install; it's way simpler. The MOPS variable should point to the directory where you placed MOPs. On Windows, make sure your backslashes (\) are converted to forward slashes (/)!
Don't put MOPs directly into an existing otls directory, or into the Houdini preferences directory that already exists. Make a new directory somewhere else.
Finally, make sure you're in a SOP (geometry) network! That's where all the MOPs are.
I'm getting some strange errors on MOPs Plus nodes like "bad marshal data" or some other nonsense!
This can happen if the version of MOPs Plus that you've installed is built with a different version of Python than the version of Houdini that you're using. Make sure that you download and install a version of MOPs Plus with the same Python as your Houdini install!
I have an active MOPs Plus subscription, but I'm getting errors saying I'm not authorized to use the latest build of MOPs Plus!
You just need to re-activate your license once a year, when your subscription renews. This regenerates your local license file, and MOPs will stop complaining.