Transaxle on Custom 600

New to this site. Just got a Custom 600 and I think it will be great. The problem is I never had a manual drive tractor before and I jammed up the gears. It wouldn't move. Removed shifter and moved gears around until I could got it in some gear (probably 3rd or 4th) Now I can drive the tractor forward but need to locate the shifter in the correct spot in the transaxle. Any attempt to change gears locks it up again.
When lowering a plow or anything the engine stalls which is why I think it may be in a higher fear. Any ideas on a solution would be greatly apprediated. Thanks