You don't need to do anything to activate add-ons. If they're listed on your account then they're already available, they fall off the account once they're fully consumed.
Your plan minutes will be used first. Add-on minutes will be used only when the plan has none available. Unused (or partially-used) add-on minutes will remain attached to the account indefinitely, month after month. While the plan minutes refresh each time you (or your AutoPay) pre-pay and renew another 30-day subscription.
You received a notification informing you about only 10 minutes left - meaning that you have the $15 plan with 100 minutes of Canada-wide Talk (not a $25+ plan with "unlimited" Canada-wide Talk) and you've used up 90 of the 100 minutes. If you use up the remaining 10 minutes (or you dial "INTL Long Distance & US" instead of Canada-wide numbers) then the add-on minutes will start being consumed.
If you plan to use (or save) the "INTL Long Distance & US" minutes for calling destinations outside Canada then you might want to purchase the $5 500-minute Canada-wide Talk add-on. It would be consumed whenever your 100 plan minutes run out but you're not calling out of country. Worth considering if you find yourself budgeting minutes and need just a little bit more each month, one penny per minute is dirt cheap and whatever you don't use it this month is still available next month (and every month after that, until you use it all up).
You can't be overcharged. If the minutes are available then they're used. If they're unavailable then the calling is unavailable (or the call-in-progress is halted). They're all pre-paid, money up front, even if the price was "$0 Free".
