Flight Question How do I release payload from SSRMS?

[Genesis27]

Hi there, I'm just in the middle of an STS-120 mission using Dave413's latest Shuttle Fleet...and I am trying to attach the Harmony payload to the ISS...
I've managed to grapple it with the arm and move it into position on the station...and I've lined up the grapple joints (one is red, and the other blue)..parent and child I think they call them....they are virtually touching and in line...but I don't know how to release the Harmony so it attaches to the ISS....everytime I click on release on the SSRMS dialog box, the Harmony just floats away...anyone know what the control is to attach to the ISS?...many thanks...