Multiple account assignment in PO

Hye,

I am trying to create a PO with multiple GL accounts

When i enter account assignment as X, item category D and enter the details, in the account assignment i want to xxxx GL account 50% and for yyyyy GL Account 50%

I tried clicking on multiple account assignment key in account assignment tab but am not able to add a new line as it is in grey mode

Can anyone please help me on this

Thanks

raj