What is the product of a Dieckmann condensation of dimethyl adipate?
I've given a strong hint by changing the title of your question!


Methoxide as a base can abstract a proton from an acidic $\alpha$-position of an ester, that is from $\ce{-C\color\red{H}_2COOMe}$. 
The resulting carbanion/enolate can add to the other ester group of your starting material.
The cyclic intermediate eventually loses methoxide to form ...


This Dieckmann condensation is the intramolecular variant of a Claisen condensation.

I suggest to do it on paper!