Can I add anything to MMF to keep it from melting when I use it with
buttercream?

More info please. Total fondant covering? Fondant accents? Is your BC of the crusting variety? Are you "playing" with the fondant and it is really warm when you apply it? Did you use enough 10x when you made it (fondant)?

Oh, dear! I've never had MMF - or any other fondant - melt when used over buttercream of any sort (American style or meringue-based). The only thing I can think of that would cause "melting" would be either water or heat. Are you using some type of "whipped" frosting? Is your MMF smooth and elastic when you cover the cake?

It happens with accents. When I cover a whole cake it works great.
When I use accents, it gets really soft or melts into the buttercream. I am useing crusting buttercream.
I am using 2 lb sugar to 16 oz bag of marshmellows.
I made a Chinese takeout box. The sides I make from MMF. Make them a couple days ahead so they would get good and hard. Put them on the cake and within a few hours, the MMF had soften.

are you using water to apply the accents? maybe it's sucking in that moisture. i like to use piping gel.

Yes, I am using water, but I have also had this to happen when I just stuck the accent into the buttercream.

Maybe you could put some kind of food safe barrier between the BC and the larger decos. For smaller, try using RI, then let sit a bit to start getting firm and place on the cake. Using water on top of buttercream just gives your decos more liquid to suck up.

Thanks, Minicuppie, I'll try that.

I think u needed more drying time or to use gumpaste. If the inside of the shape us still soft, it's more likely the whole thing will go soft when it's put on bc. For something that sounds like a large shape, it would need longer to dry properly.
U only need the tiniest bit of water ( or whatever!) to make fondant stick on.
xx