Under the "Program" tab, select "Database". That should pop-up the program window on the "IR Data" page. Under the "Devices" drop down, select your Roku player. Then on the "IR Data From" drop down, select AUX, and then type 839 in the "Enter Setup Number" field to the right of that drop down. Then click "Save All" to pre-populate the Roku Device page with all of the IR functions that URC has under the AUX 839 codeset. From there, you can move the buttons around, if you don't like the default layout, under the "3. Edit Buttons" tab within the Program window.

Alternatively, if you don't want to pre-populate the Roku device page with all of the AUX 839 presets, you can add individual buttons as desired by selection "Edit Buttons", type select the roku player in the "Device" drop down winder, and then enter in the name that you want for the button. Then exit click "save" and exit out of the program window. Under the "Model Designer" window on the left side of the screen, click on Page 1 of the Roku player. On the right side of the screen, find the "IR Database Navigator" tab, and then type in AUX 839 in the "IR Data From" drop down. From there, it should populate all the available codes under AUX 839. You can simply drag and drop the command to the button label you created above. (Or click on the button, and then drag and down the command into the "Macro" tab on the bottom right of the screen.