Call UA for seats. Note though, that the PNR that AC gave you (or that is on your itinerary on the AC site) will be useless to them - UA should have their own PNR for you. In fact, might be a good idea to get this, as you can check your itinerary in future on the UA site by conf. #/last name. However, since it is likely an AC flight number, you probably won't be able to choose a seat online, only by phone.
Only UA elite's are entitled to free E+ seats for about the last 2-3 years (asked and answered many times in about tens of threads). You definitely won't get them by calling, ICC agents definitely won't do that. You can ask with the agent at check-in or at the gate about E+, but likely won't be given for free, unless the flight is heavily booked, in which case you probably have a better chance.If you really want E+ and are willing to pay, you can try logging in to the United.com with your UA PNR and see if travel options are available for E+ (this is available for UA, just not sure if you can do it on an AC-coded flight). Price will vary based on flight length, and I haven't looked at this in a while, but I think it used to be either $14 or $19 for E+ on YYZ-ORD ('bout two years ago).