DBISAM does not support using DISTINCT in COUNT aggregate queries. If you want the same functionality, however, you can get it with this SQL script:

SELECT DISTINCT MyColumn INTO "\Memory\Temp" FROM MyTable;
SELECT Count(MyColumn) FROM "\Memory\Temp";

Note
Be sure to drop the Temp in-memory table after you're done with the result set. You can't do it inside of the script, however, since that will remove the result set.