In the left hand navigation bar choose Change in Residency, choose the button beside Yes my residency has changed.  Once you do this four fields become visible.  Enter the date of your entry to Canada.  Then enter the amount of money that you earned for the part of the year that you were not a Canadian resident.

I've done that.  But it seems I need to also enter the foreign income as a foreign slip under "T slips" and then again to claim foreign tax credit.  However, when I do both of these things I still get an error when I review it, and the amount of tax I suddenly owe is in no way correct.

The only income that you need to enter under Foreign income under T slips would be the foreign income and taxes paid after you entered Canada.