I've tried to make this frosting 3 times now.. I just cant seem to get it.. Heres what I did..
I mixed 1 cup of butter with 1 1/2 cups of powdered sugar, 1 tbsp on vanilla extract and 8 oz of marshmallow fluff.. Ive beat it for 15-20 mins and its still creamy... almost runny.. So i tried adding an egg white.. still no change.. Please someone help!! what am I doing wrong?

With that amount of fluff, I think you may need to increase everything else.

The marshmallow bc recipe that I'm in love with right now is by Mrsmudrash...it's on her blog, under "What buttercream do you use?"
http://jessicakesblog.blogspot.com/2011/05/most-commonly-asked-questions.html )

1 lb unsalted butter - whip for 10 min on high
1 - 7oz jar of marshmallow cream - whip on high for 5 min
1 lb powdered sugar - whip on high for 5 min
1/4 cup heavy whipping cream - whip on high for 5in
3 Tbsp vanilla - whip until incorporated.

This makes a really light amazing buttercream. You can add more heavy whipping cream (the liquid, not actual whipped cream) if you want, depending upon the consistency you're looking for.
*You can also throw in a block of cream cheese at room temp to make it even more yummy!

Oh.. See what I did was put in all in a bowl and mixed it... no pre mixing at all. Could that be the problem?

I still feel like it's the ingredient amounts, but I'm definitely no expert, so I could be wrong!

The shortening based bc that I also use, I've done both ways...beating the heck out of it in stages, and also dumping everything in with almost no mixing in-between...same result.