The DACs on the 888 are 0-5V. You change its voltage by using the SETDAC command and specify a value between 0 and 4092.
The DACs are high impedance outputs and are not meant to directly drive any load that demands more than a few micro amps of current. You need to add buffer amplifier (OpAmps) in order to drive loads. The OpAmps also allows you to change the voltage into other industrial interface such as 0-10V.