Hmm, weird.

Instead you can add that line to your system's environment by going:

1. Right click on My Computer and go to Properties
2. Go to the Advanced tab
3. Click on Environment Variables
4. Down in the System Variables section, click New
5. Put the line MAYA_MRFM_SHOW_CUSTOM_SHADERS into the Variable Name slot
6. Then just put the number 1 into the Variable Value slot

Then restart your computer. That may fix it for you.

Oh but I think you should take that line out of the Maya Environment file if you do it this other way.