If one of the fields is Null (blank), the answer will be Null.
Use Nz() to indicate to use zero for null, i.e.:
Value3: Nz([Value1],0) + Nz([Value2],0)
If that doesn't work, tell us about the data type of these fields. If they
are Number they should add, but if Text you may need to typecast them.
Details in:
Calculated fields misinterpreted
at: http://allenbrowne.com/ser-45.html
I hope you're using & and not +. It might be problem with field
data types or you're just missing some operator in the field. What's
the error message?