If you have tecplot 360, not focus, Go to Analyze menu and select calculate variables then select cell volume, select cell center for variable location. That will calculate the cell volume for each cell you can then do a summation outside of tecplot for the areas you want or
use data alteration to assign blanking values (integers) to each area of the grid in which you want to calculate the volume,
then go to plot/blanking/value blanking and then select a blanking value and set it not equal to the area that you want blanked
then go to analyze/perform integration and select over cell for integration and turn on excluded areas that are blanked. If you have a lot of different areas in the grid to calculate volumes this could be tedious.