The design of the transmission set-up has changed drastically since this mower was new. Because of that, I don't seem to have a photo of how that spring and retainer are set up on the mower.
That set up sits on the front right side of the transmission. It's basically there to hold the transmission straight but allow it to have some movement to it to help keep the transmission from binding up. There should be a hole on the front right side of the transmission that the retainer pops into.
If you do not feel confident repairing this problem yourself, then you can have it repaired by a Sears technician. Here is a link that will provide the nearest drop-off locations:
Sears Home Services