In an nMOS, how does applying Vdd to the gate attract electrons forming a channel?
As part (b) shows, when you apply a positive voltage to the gate, then it will have a positive charge.  This positive charge attracts electrons to the area so that they can make the "jump" from drain to source.  

Note: The electrons would flow to the gate if it weren't for the "Metal Oxide" in the Semiconductor layer, which insulates the gate from the substrate and the source and drain.

This area in the substrate where the electrons are attracted is called the channel.  The more positive voltage applied to the gate (higher voltage), the larger( stronger potential and larger in size ) the field in the area known as the channel.  Thus allowing more electrons to flow from the drain through the channel and to the source.

PS. I know that electrons actually come from the source, but we like to think of things in positive terms for simplicity.  The math is the same.