need help on drop down list

Iam trying to create the drop down list. Well, I got some programs, creating these lists for both parameter and ranges on the selection screen.

What the user wants is when the selection screen is loaded, he wants to select from the list OR ENTER ANY DATA WHICH IS NOT THERE IN THE LIST. To be able enter any data other than what is there in the list is not working for me.

itz urgent plz