Loading the Plug-in

You can either load the plug-in from within the (MEL) Script Editor (providing the full path to the DLL/SO):

loadPlugin "/mill3d/users/jan/git/arnold_shaders/maya/2016/assExport.so"

Or you can open the Plug-in Manager:

And navigate to the DLL/SO on disk by pressing the Browse button:

But regardless how you loaded the plug-in, once it is loaded, you can press the green info button to get some Plug-in Information from Maya:

Alternatively you can ask for the same information via MEL (or Python):

pluginInfo -query -command assExport;
pluginInfo -query -translator assExport;
pluginInfo -query -dependNode assExport;

Unloading the Plug-in

This becomes important if you try to unload the plug-in. As soon as you did create one of the plug-in specific dependency nodes, you will have problems to simply un-tag the Loaded entry (to unload the plug-in):

With the command above you can actually ask for the names of all dependency nodes the plug-in potentially creates. So with that info you can automate the removal of all plug-in specific nodes:

// dependency nodes
string $cmd = "delete `ls -type %s`";
string $itemList[] = `pluginInfo -query -dependNode assExport`;
select -d;
string $results[] = executeForEachObject($itemList, $cmd);

After that you should be able to un-tag the Loaded entry, or from MEL you can use this commands (you have to flush the undo anyway before un-tagging the Loaded entry):

// unload plugin
flushUndo;
unloadPlugin "assExport.so";