Push down to tension the chain. Rotate the crank pulley a little clockwise (by hand) before you press down on the pin (that way all the chain slack will be at the adjusting guide. Don't go nuts pressing down on the thing real hard. There is supposed to be some slack/clearance. I would press down to take up all slack then back off the pressure to tighten the bolt.

-Todd
1987 Montero 2.6l 5-speed (Sold...but I still miss her)
2008 Jeep Patriot Sport 4x4