All the fields in your table are of type TEXT. Basic operations (e.g string operation) on TEXT data is not possible. You should should convert the column types such as TEXT to VARCHAR(250) for slsmName column, TEXT to FLOAT for NetAmt column and so on. Otherwise, DB can not calculate SUM (TEXT data). These datatype change operation may throw error. Let me know what are they. I shall let you the fix for them.
Anyway, you may use the following query to get intended result. But this will run once those type conversion are accomplished.
SELECT TOP 10 slsmName, SUM(NetAmt) AS TotalAmount FROM salesDetails WHERE slsDate BETWEEN #15/3/2013# AND #5/17/2013# GROUP BY slsmName ORDER BY TotalAmount DESC