a simple quetion: how to activate UserExit

Hi all, i must tell you that my english is not so good, but i'll do my best...i'm new in SAP and programming in ABAP. I know that there are a lot of answers about User exits but i need the simplest you can....

In this i need an Exit for VF01.

I have ZFindExit, when i click in an UserExit, for example SDVFX011 the transaction SMOD appear. then i enter to the function module EXIT_SAPLV60B_011 and the include ZXVVFU11 appear, but when i do double click, a yellow icon appears at the bottom of the screen and i cant enter the include to put a break for example,

can somebody help me.

i promise that points will be rewards!!!!

thank in advance!