If you machine is equipped with the valve it will be located just below the seat in front and just above the shifter, The selector valve should have a "T" shaped handle on it and an arrow on the handle points to the direction of the Hydraulic oil flow, meaning if the arrow is pointed inward(towards the left when seated) then the flow is going to the 3 point hitch lift mechanism, If the arrow is pointed outward(towards the right when seated) then the hydraulic flow is going to that auxillary port and anything that is hooked up to it, I will caution you not to turn the valve when the tractor is running as doing this can damage the hydraulic pump or rupture a hydraulic line or at the least blow a gasket or seal, If the arrow is facing the left meaning that it is working the 3 point lift then to switch it to operate the auxillary port you need to first place the 3 point lift handle to the rear position(this would be the lift position) after doing this then you can turn the handle so that the arrow faces the auxillary port to operate your dozer blade or whatever you have hooked to it.Then you can start you tractor , The 3 point lift handle must remain in the pushed to the rear position for hydraulic oil to continue to flow and if you return it to center then the auxillary port will not operate.
Below are some pics of the valve on my B5100E, It is the same valve and I added it to my machine and sourced it from a B7100