We've had a batch of covers that unfortunately were not made very well and the hole in the cover doesn't align well with the charging port on the tablet.
Here's what works for us:
First you'll need all of these things:
It's a lot easier to plug in the usb charging cable if you do two things
1) Mark the included usb cable with some sort of color so you know you're putting it in right side up. For example, put a green mark on it and cover it with tape (otherwise the green will simply come off on your fingers when you use the cable
2) Then flex the foam-rubber cover enough to give the charging port a little more room while you push in the usb cable: