DLookup("[Factor]","Mon","[Difference] = " & Var1)
This assumes you have already set the value of var1 before getting here. It doesn't really matter what data type Factor is
If difference is numeric then the expression would be
"[Difference] = " & var1
If text
"[Difference] = '" & Var1 & "'" start with double quote, single then double quote in the middle set and double, single & then double at the end.
If a date
"[Difference] = #" & var1 & "#"
Kelvin