and the cake is for tomorrow. I followed the Waldorf Astoria recipe to the letter. and it's way too thin to put on a cake, please can someone tell me what to do to thicken it enough to use? tastes awesome and I hate to just dump sugar in there if that's not the best option

I'm not familiar with the Waldorf Astoria recipe but I use this one and it's perfect:

http://www.bakespace.com/recipes/detail/Ermine-Frosting-(Butter-Roux-or-Boiled-milk-Frosting)/13589/

Is yours too warm? You may need to chill it. When I make this I let the flour mixture cool to just about room temperature and add butter that's still cool, not room temp or soft.

that's what i did. will sit it in the fridge a  bit and beat again. pray for me....
 

what a failure. I made another batch of icing. it just wouldn'[t stick to the cake, kept sliding off. I put everything in the fridge overnight and when i tried to rebeat the last batch it curdled. going to have to make some creamcheese and slap it on there real quick. so disappointed and wasted a pound and a half of butter on 3 batches. never again.

Hope you get the one that you are expecting!

3 tablepoons of flour is a minimal amount.  When the flour is cooked in the milk, the result should be a thick paste-like mass.

Check this blog for some good photographs. http://tastykitchen.com/blog/2010/03/a-tasty-recipe-thats-the-best-frosting-ive-ever-had/

It must be totally stone cold before adding it to the butter. Any heat will melt the butter ...