Jessie,

The Census search option for 1840 is Benton County. It is also Benton County for 1850. The search option changes to Calhoun in the 1860 Census.

For the individual search it allows you to enter Benton as the location. I use this all the time as this is one of my main research areas. Granted, the suggested place name is something else. But, you still have the choice to enter Benton. (See the screen captures). The search then finds the records in Benton County.