MO2 looks for specific file names. No matter what you install, if that file name isn't in the right pane and checked (activated), it will throw that Missing Master warning. So, there are a few possibilities:
A) You haven't found the actual mod you used originally. Keep looking.
B) You found it, but the author has changed the name of the ESP file. Depending on how extensive the other changes in the mod are, you MIGHT be able to rename it to the file name that MO2 is looking for.
On a related note, where, exactly, is this warning being thrown? The only place I can think of is as a missing master for another mod (red triangle next to a file name on the right pane). Depending on where it is coming from, there may be other options available as well.