Heya, this is quite simple.
1. Enter 2 textboxes (one for the height and one for the mass). You shall name them as in your script: weight, height
2. set up a local or global variable (up to you, what you intent to do) which will record the result.
3. in the event sheet you can then set the value of the variable to: Math.round(weight / (Math.pow((height/100),2)))
4. add an event to check the result and output what you want to a text object
EDIT: Okay, for step 3, you have to change the math a bit. Attached is an example