It's possible you've been embargoed by that player. If you can't get into orbit (on the circle surrounding the planet) than it is either a glitch or you've been embargoed. But I couldn't say for sure, never tried trading with a player that has me embargoed and never had an issue getting into orbit.

The way trade is set up is: Send your Harvester/Cargo ship to the colony planet of the other player you want to trade with. Once it has arrived you should be able to click on your unit, when you do, you should see a new icon on the right of the "Harvest" Icon. When you click on that, it will ask you to choose your colony planet you want it to create the route with. Once you select that, your job is done. Remember though, that you can only have one route per colony planet you own. So multiple ships can be at the other player's planet, but each must be going to a separate colony of yours.