Your box will be a solid gear one filled with grease.
The grease gets hard with age and makes changing hard.
Pull it out, clean it thoroughly on the outside & pull it apart.
Usually the top & bottom just lift off.
The gear change lever will have a ball bearing & spring that hold the gears in place when you change be carefull that it doesn't jump out an go hide down the back of your workspace.
You am find your manual here
http://gardentractortalk.com/forums/files/ but you will have to sign up & Join the forum to access the files.
Some of them are on the site & others are just links
Next thing to look at is the linkages.
If it has been hard to shift for a while then the holes will have flogged out oval which makes shifting even harder.
On a mower of that age it will be a case of welding the holes & reforming them.
If you have a stick shift box then just lubing will usually free it up.
