Rate of exchange on journals

On AX2012, when creating a payment journal for customer receipts, in my test system (with demo data), and when selecting a currency that is different to the legal entity currency, I can amend the rate of exchange that is populated from the rate of exchange table, thereby using a specific rate of exchange for this journal line.

Now I have a new environment set up, but when I go to the payment journal, and try to duplicate this, the rate of exchange field is greyed out and not available for editing.

I’ve obviously not set this up correctly, but can’t for the life of me find where the setting is to control this.

Any idea’s anyone ?

Thanks Andy