Assuming this is the Canadian Amex Platinum ...

You don't get MLLs in Canada, as there are the Plaza Lounges, but you get MLLs in the States and International through Priority Pass.

There are 3 ways this card gives you lounge access:
- Amex Platinum card itself
- Priority Pass
- Marco Polo Gold card (OneWorld Sapphire)

You're allowed at least 1 free guest with any method. The Priority Pass the card issues is also eligible for the United Club (the U.S. Amex version excludes this).

Also, any supplemental cardholder who gets an Amex Platinum is also entitled to the full Priority Pass card (but not Marco Polo). I got one of these for Mrs. YVR-in-law and she now has global lounge access, regardless of airline or class of service for $175/year!

After my year is up, I may consider switching to the BMO World Elite. Costs a lot less, no supplementary card lounge privileges, but it comes with a Priority Pass card with 4 free visits. For someone with *Gold lounge access, that may be enough to fill in the gaps on the odd time you need it.