Go into Configuration Editor, select modular HPLC system (I think you've already done this). Select type of 35900 you have, such as 35900C. These typically have the address code of 13, enter that; click boxes for "A" and for "B", then OK. The listing for the A/D 35900 should now show up in the box, so go to File, Save. Then Print, and exit.
Open the Chemstation. You should see a green "ready" icon labeled ADC either under or over your VWD or other detector icon. You may need a remote cable attached from the 35900 box to the stack.
We run ELSD and conductivity detectors on 1100 systems with external A/D boxes (ours are 35900D) and on 1050 systems with internal 35900 cards.