Rebecca,
First, transportation. You should plan on waiting for a bus at least 15 minutes at Downtown Disney.
After that there are two ways to get to the Polynesian. The first is to take that resort bus. I would expect about a 25 minute ride (assuming a stop at the Contemporary, which can happen on occasion). But if a bus for the Magic Kingdom comes first, take it. They typically come more often, and you can go from the Magic Kingdom to the Polynesian either by boat launch or monorail.
For dinner at Downtown Disney you should consider not using the Dining Plan. Many of these restaurants either are not on the plan, or have limited menus for people redeeming the dining plan. Usually when using the dining plan you find yourself 'spending' all your credits before your trip ends anyway, so you may as well pay for a meal here.
That being said, I would recommend
Raglan Road (Irish),
House of Blues (Creole/Cajun/Southern), and
Bongos (Cuban). They offer great cuisine and wonderful ambiance. Raglan Road (my favorite of all), does accept Disney Dining plan credits.
Have a magical (and delicious) vacation!