I believe that MS Access uses the Microsoft Jet Database Engine and that the countd function is not supported. This is true for Excel and text files too. So, to get around this limitation, many Tableau users will extract the data (right click the data source in the left window under "Data" and select "Extract Data..." When you create an extract, the data will be pulled into Tableau Data Extract (.tde file) which is a proprietary database that Tableau can use. It's a very fast and effecient database that supports some of the functions that the Jet Database Engine does not (such as COUNTD).
However, once you've extracted the data, you are no longer looking at the live data -- you're looking at a snapshot. So, you won't see changes in your Access data reflected in Tableau until you update the snapshot. To update the snapshot, you'll need to refresh the extract (right click the data connection select "Extract --> Refresh".
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The extract solution is the easiest, it can also be possible to do this via table calculations or RAWSQL expressions, see this thread for details: http://community.tableau.com/thread/109935
Thanks Joshua - this worked for me.
Thanks Jonathan - I think this should work but just can't seem to get the counts I'm looking for. I'll use the extract on this one.