Negative charge of methoxide anion
The negative charge is due to the unpaired electron in the oxygen atom, remained after all bonds(molecular orbitals) are filled. Oxygen has 2 unpaired electron in its valence shell and is singly bonded to carbon in this molecule. Hence being un-bonded, the other electron exhibits the charge.          

Check this link, hover the cursor on the $\ce{O}$-atom, you will get a graphical representation of the same.