Right, well at least we can solve the problem. Two problems actually.
Firstly, go and read my tutorial
macguba's guide to mission editing[/url] which will help make things clearer.
Secondly, the player unit is the unit which is controlled by you rather than the computer. Double click on the unit you want to play: click on control (on the left) and now select player. You have already done this once because the first unit you placed on the map was a player unit.
Thirdly, look around your map. You will see pale blue lines connecting some of the units. Connected units are members of one group. The maximum number of units in a group is 12. Somewhere you have a group with more than 12 units. Click on Groups at the top of the screen and drag and drop the thin blue line from one of the excess units either onto an empty space on the map, or another unit in a different group. It sounds much more complicated that it is, just play around. (If you have tanks or vehicles the game counts each crew member as one, so four tanks is the usual maximum for a group.)
Now you should have a Preview bottom on the right hand side.
Oh and
Welcome to the forum!