It sounds like the Field Size you've chosen for your field is Integer,
Long Integer, or Byte.  These are all whole-number data types, and can't
hold decimal places no matter what format you pick.  Change the field to
Single, Double, Currency, or Decimal, as appropriate for your use.
Single and Double are both floating-point types, which means they store
an imprecise representation of numbers, but their precision is high
enough for most purposes.  If you need absolute precision to a certain
number of decimal places, you can use the Decimal type, but Access has
historically had a few quirks in handling fields of this type and I'm
not sure what the current status of the Decimal data type is.