Problems in Assigning PDF form in dunning procedure

Hi Gurus,

I have created a z adobe form for dunning letter and would like to assign it to the dunning procedure in Tcode F150.

I have gone through the path on SPRO.. Financial Accounting (New)->Accounts receivable and payable->Business Transactions->Dunning->printout->Assign Dunning forms..

In here it shows standard SAPscripts and I would like to change them and put the adobe forms on different dunning level..But now this is greyed out and does not allow me to add my own forms .

Does anybody have an idea on why this is the case??