EXIT through OB28 for GL check

Hi Guys,

I have a situation where i need to stop user from posting document which is having S type (S1XXXXXXX) and L type (L1XXXXXXXXX) GLs together in line item for certain company code. i have to do it through OB28.

where do we mention exit names like U001, U201 etc in OB28.

if i click on abap source code icon, it takes me to RSBBXXXX not to its Z-version (ZSBBXXXX). what settings are required for it..

Thankx a lot in advance.