Which Grafana version do you use? Which plugin version do you use? Did you use the latest version of our documentation?
My Grafana server is running on localhost:3003 and I can see the plugin here:
http://localhost:3003/plugins?filterBy=all&filterByType=all&q=checkmk
In the Grafana logs you should see a line similar to:
logger=plugin.signature.validator t=2023-05-31T12:27:51.613245706Z level=warn msg="Permitting unsigned plugin. This is not recommended" pluginID=tribe-29-checkmk-datasource
logger=plugin.loader t=2023-05-31T12:27:51.613263224Z level=info msg="Plugin registered" pluginID=tribe-29-checkmk-datasource