Customising Dimension Security

HI, I'm having problems specifying customised security in the Advanced tab (allowed members). I'm fairly new to mdx script and am trying to restrict access to a STAFF dimension which contains a loginname member which I want to restrict access to based on username matching the loginname DIM. Have tried the following which didn't work:
[STAFF_DIM].[Login Name].members(MID( USERNAME, INSTR(1,USERNAME, "\") + 1, 128 ) )

I read somewhere that this can only be used on leaf layer members (this is the first of 3 layers), also should I be using the filter function? Any help with trying to setup would be great help.
Before using this expression for dimension security, you may want to do some experimentation first, by creating a simple MDX query including this expression, and verifying that it evaluates to the expected member...

Hope this helps