I'd do something like this (syntax could be way off-base but you'll get the gist of it)
Dim FMLAOn As Boolean
FMLAOn = Me.[FMLA] '(or Me.[FMLA].Value) I am rusty so I have to try both and see which is right.
Me.[Occurrence].Enabled = FMLAOn + 1
I am sure there most be a more elegant way to do this. But I think this works. If FMLA is checked (-1) then adding 1 changes the boolean value to false. If it's not (0) then adding 1 makes the result non-0 and again sets the boolean value to true. (Isn't that right, any non-0 number evaluates to true in Access?)
If I'm getting my languages confused, you'd have to substitute an expression to invert the value of OccurrenceOn before assigning it to the Enabled property of [Occurrence]. There's probably a function to do that too.
You could always do this:
If Me.[FMLA] Then
Me.[Occurrence].Enabled = 0
Me.[Occurrence].Enabled = -1
Attach this or similar code to the appropriate change event in the FMLA field and you're set.