You usually don't have to prompt the database optimiser for index utilisation. The database will automatically utilise these indices if the database thinks it's the most logical way to obtain the data.
So, if the index is made up of 3 fields, but you're only selecting 2 of the 3, then the index won't be used.
If you want to force the query to use the index, then you'll have to use SQL or a stored procedure as your datasource. Alternatively, you could select the remaining field that you don't need so that all requirements for the secondary index is met - and so it'll be used. Naturally, you'd want to suppress the extra field.
All the best,
Naith