Hi Sarah!
You must be getting very excited for your upcoming trip - it's almost here!
I would think that you will have plenty of time to make it to your dining reservation on time as long as there are no flight delays or unforseen circumstances which would delay you. (I'm knocking on wood!)
The amount of time it can take to get to your resort from the airport on Magical Express can vary, but I think a good idea is to estimate about an hour from the time you check in at the ME counter until you get to Pop Century. The airport is about thirty minutes from Disney World, and the busses sometimes stop at multiple resorts, so you should expect to be at your resort sometime around 11:30 or so, give or take a few minutes.
When you arrive at Pop and check in you can leave your carry-on bags with bell services if your room isn't ready yet. They can deliver them to your room later in the day for you.
Starting at ten days prior to your arrival you can check-in online which will expedite the process for you once you arrive at the resort. You can access this feature
HERE.
I hope this helps! Have a fantastic vacation!