I have no idea why this happens,,,,but it is totally reproducable.

once you download imanager 27 workstation(may have with server
version),,,,if simply select all the plugins to install and then go away
for a bit. the IDM plugin will not show up. Nothing I can do can make it
show up.

The only way I can get the idm plugin to work is to install it as the
FIRST plugin. Nothing else selected. Then you can go about installing all
the other plugins.