Can't send to mix outs on RML16AI with Studio One 3

I really hope someone can help me out here. I have a CS18AI that I use with an RML16AI into Studio One 3. My problem is that I can't work out how to send cue mixes to the mix outputs on the RML16AI. In fact, I can't even see the outputs in Studio One 3. I can set up sub-outs in the I/O config and see signal on the associated meter but how do I get it to a physical output? Any help appreciated. Thanks.