How to equip Keen's ninja stars?

You cannot equip the throwing weapons Shadow the ninja sells. They are instead used in battle like consumable items. Just choose items with Keen, scroll down the list until you find them, select the weapon, then select the target.

If I could have I would have made the damage from these items scale as you character levels, but that's not really possible at this point and much more difficult to do than with skills. The better stars become available as you progress through the game. Check back with Shadow every now and then to see if he's updated his stock. Also, while they may do lower damage than attacks, these items ignore the enemy's defense. So if you encounter a foe with really high defense and physical attacks are not effective, use some of the throwing weapons to cut right through them.

To check your version # just look in the lower right corner when you load the game on the title screen. You'll see "v3.2B" or "v2.16" or something similar. I'm assuming you're playing the new version and am going to move this to that board's discussion section. That's where people should be posting/looking for answers these gays anyways, so if someone else has the same question as you did they won't miss it there.