I have the same question. Did you get an answer? 

Not yet. If I don't get an answer by the 15th I'll just mail in our T4's this year. 

I called my Sage rep. He said just use the number 1 or use your myCRA Represent a Client ID. Guess what? Neither worked when I tried to submit to CRA online. I will be calling CRA tomorrow to see what I'm missing. Also.. I'm pretty sure you HAVE to file electronically if you have more than 5 slips. There are other ways to do it, like manually enter the info. Anyways, if anything interesting comes out of tomorrow's call to CRA, I will update here. 

Thanks for the tip on the more than 5 slips. I tried the trick and used the number 1 as the Client ID and it worked. I hope you're able to get some clarification when you call CRA

No problem! This might be good info for all. In Sage, on Transmitter Info II tab, where it asks for Transmitter Account Number, choose option BN15 account, and then do your business (payroll) number followed by RP0001. If you are representing a client (my employer in this case) you would enter your Rep ID here. If you aren't repping a client, just put the number 1 (as above comment states). My xml file wasn't being accepted originally because I had chosen BN9 and was only putting in the 9 digit business (payroll) number. Hope this helps!