In the acid-catalysed formation of a dimethyl acetal from an aldehyde RCHO and methanol, the species that is attacked by the second molecule of methanol is

The hemiacetal is protonated on its OH group and loses water, assisted by the lone pair on the other oxygen, to give the resonance-stabilised oxocarbenium ion $\mathrm{R}-\mathrm{CH}=\mathrm{O}{}^{+}-\mathrm{CH}{}_3$. This electrophile is attacked by the second methanol, and loss of a proton gives the acetal. The medium is acidic so no alkoxide is present.