user exits

hi ,

Please help me out in writing the user exist.

Here i am mentioning the step by step process what i am following.

1) In cmod t-code i am creating the z project.

2) assign enhancement component ( that is exit name FEDI0003)

3) In change mode double click on FEDI0003 and it will take to the function module

4) In FM i have one include zx_____ , when i double click on that it is asking for the access key

5) I am assigning the access key to that then it is showing one popup saying that made changes if it is urgent. Here when i say yes it is not allowing me to write the code and it is sending message saying that source text can not be change

Regards,

kumar