Access Pivot Table HELP!: calculated columns

There doesn't seem to be much information around about Access Pivot Tables,
and I don't do much in Excel. MS Help is no help whatsoever.

I'm trying to create two columns: average units and average price, and
everything I try comes up with errors. One manual says I should be able to
click on a column (Units), then click on Autocalc and choose a function (like
Avg), but when i click on a column, Autocalc is grayed out. ????

Then if I try to create a calculated field, detail or total, I get the
Properties/ Calculation box, and have used Insert Reference to create a
calculation (NetCharge / Units). However, the results are horribly
inconsistent: sometimes the calculation is correct and sometimes not. (I
have to perform this calculation at a total level, because some entries may
have a charge but no units, as when a charge adjustment has been made. I
can't do the calc at a detail level in the underlying query/table.)

Very frustrating, very confusing, for something which has the potential to
be a very useful tool.

Any help gratefully appreciated.

Kevin