Hi there and welcome to the Esterbrook family.

There is a small opening on the back end of the nib section that allows the ink to flow down into the feed. It is located near the edge and will be on the same side as the nib itself is located (opposite from the feed side).
If the pen wasn't rinsed well after its last use, you may have to rinse the nib off in order to see the opening. Feel free to run the entire nib unit under some cool water before screwing it back into the pen. If it is very inky, you can soak the entire nib unit for awhile. In addition to drawing water through the nib, I often soak mine for awhile just to keep my nibs clean.
What kind of nib did your pen come with?
Todd