If the key won't come free, you can partly disassemble the hub far enough to expose it, without taking the whole thing apart.

Stand the wheel in a vise, remove the right side locknut, and use some nail polish to make reference marks on the tabbed key washer, cone, and axle. These aren't really necessary, but may help with your confidence.

Remove the tab washer, and right cone, and the sprocket and driver assembly will lift off, so you can see the cross shaped clutch and the key. Now it's easy to tap either end of the key to dislodge it, and once free, to attach the indicator and work it in and out until it moves smoothly.

Reassemble, and adjust the right side cone so there's no play in the hub, and/or by the reference mark. Add the tab washer and locknut. Finish by checking for axle/wheel play the same as any wheel, and complete the final adjustment from the left side.

Since out mutual friend won't let me delete this post, please proceed slowly and carefully, so you don't have any reason to regret following my advice. But seriously, this is child's play for anyone who's picked up a wrench more than once or twice.