The option seems grayed out:

However, Google seems to indicate that it's possible:

Aliant
Bell Mobility
Fido
Microcell Telecommunications Inc. (Fido)
MTS Mobility
NorthernTel
Rogers
TBAY Mobility
Telebec
Telus Mobility
Virgin Mobile


• – Thufir Apr 13 '13 at 0:33
• – Thufir Apr 13 '13 at 0:43

I used the mobile reminder feature in four different European countries, I'd be surprised if it wouldn't work for Canada.

Your screenshot is from the settings for one specific calendar, you need to go to your general calendar settings to associated the phone number:

-> Options button (right side) -> Preferences

There should be four tabs (General, Calendar, Mobile, Labs)

-> Mobile

enter the number, verify and then go again to the preferences menu for this specific calendar

-> "Calendar" tab

-> TUFIR HAWAT (or any other calendar name)

-> Reminders

the option should now be available