You shouldn't ask us how to do this. You should have shown us your attempt to do it and they asked us how to fix it.

sum = 0

For i = 1 To Val(Me!Text0)

sum = sum + i

Next

but you don't need to use a loop it's just

n(n+1) / 2

5(6)/2 = 15 <--see

if you wish you can check out the docs on loops and stuff http://msdn.microsoft.com/en-us/library/...