||From the help files under "Area/calculating for polygons'
1. Optionally, start an edit session in ArcMap. Calculating a field is faster outside of an edit session, but you won't be able to undo the calculation.
2. Open the attribute table of the layer you want to edit.
3. Right-click the field heading for area (if there is no field for area values, you can add a new field for area by clicking the Options button and selecting the new field option).
4. Click Calculate Values.
5. Check Advanced.
6. Type the following VBA statement in the first text box:
Dim dblArea as double
Dim pArea as IArea
Set pArea = [shape]
dblArea = pArea.area
7. Type the variable dblArea in the text box directly under the area field name.
8. Click OK.
Hope this helps,