How to calculate the theoretical yield of a Diels-Alder reaction? A Diels-Alder reaction happens between 9-Anthracenemethanol (208.26 g/mol) and N-Methylmaleimide (111.10 g/mol). 

I'm using 0.069 g 9-Anthracenemethanol (0.033 mol) and 0.11g N-Methylemaleimide (0.0009 mol, limiting reagent).

Since DA reactions have 100% atom economy, the product has the molar mass of both products combined, 319.36 g/mol.

Multiplying that by the moles of limiting reagent, we get 0.28 g product. 
However, the mass of both starting materials combined (0.069g + 0.110g) together is not even 0.28g. What am I doing wrong?
You are missing a  couple of zeros in the number of moles of your 9-anthracenemethanol.  I calculate 0.00033 mol of that reagent, which therefore becomes your limiting reagant, and I calculate a total yield of 0.105 g of product, with about 0.73 g of  N-Methylemaleimide left over.