I need to set an esp file as a master for my mod, but I just can't get it to work. Can someone help me?

I use 'TES4Gecko', I think its v15.2 or something.
Anyway, its made for Oblivion, but it works just as well on Fallout 3 mods.

Its easy to use. You start it up, click the button 'Convert to Master' and point to your ESP. It makes a copy as an ESM.

I'd suggest using http://www.fallout3nexus.com/downloads/file.php?id=637. Open your plugin and the master you want to add, then R-Click your plugin in the left panel and select "Add Masters", select the desired plugin, then R-Click "Sort Masters" (only needed if the plugin already has more than one master), close, save.

Alternatively, you could tick the master you want to add in the GECK, then load your plugin set as active and the GECK will add it to its masterlist (potentially out of order though) when you save.

Thanks