exices invoice and subsequent exice credit to company

the process flow for in my company is PO-- GR exice credit (j1iex) miro -- reconsilation of exice credits. i have already maintained the exisice master for a particular material and vendor.

But iam not getting exice pop up in PO (or) GR. can anybody answers below question.

- why the exicse pop is not coming in PO/ GR.

-can any one give the total configuration steps for the above process.