An application that runs on 2 PC's cant talk through Actiontec router

I have the Actiontec M424WR Rev1.

Internet access for the two PC's  works fine.

I have an application that lets 2 PC's share a mouse and keyboard. It doesnt access the internet.

On a linksys router without internet  it works but Not on Actiontec.

I tried port fowarding

(which i understand is to  out or in from interent) still no good.

i have rebooted the PC's and Actiontec

any ideas ?

thanks