If you want the item to add the bonus whenever it's in the player's inventory, you should use the global script page and ITEMSTATE commands. (Something like this: "IF $ITEMSTATE1 = 1 THEN %STRENGTH := %STRENGTH + 1")
If you only want the item to add a bonus when used, you could put a similar string of code into the item's individual script, but this would be trickier to smooth out with the rest of your variables.
I recommend re-reading all of the advanced scripting articles (especially the one Ogre posted), and then playing around with the global script and item scripts until you're familiar with how you can use them. Then, you'll be better able to figure the most efficient way to customize item-stat modification for your specific game.