how to get the P4 to revert unchanged files before submitting?

[steve]

Hi,
When using a Perforce action, I cant set it up to revert any unchanged files before submitting them. The checkbox in the Perforce configuration tab is there, but never seems to be enabled no matter which command you set!

Is there something else that needs to be done before the checkbox is enabled?

BTW, I'm using 5.2

Cheers,
Steve.