My old tank was rusted and got a hole in it... so I installed a bladder tank... now the new bladder tank won't fill with water... what do I do?

the new bladder tank wont fill with water
Bladder tanks don't fill up, (see image) If you ever have one that fills up you have a bad one.
Let me explain the function of a bladder tank. Pressure and bladder tanks provide a air cushion that produces water pressure in your system so your pump doesn't kick on every time you draw a glass of water. A bladder tank has a neoprene membrane in the center. As the pump puts water into the bottom half of the bladder tank the membrane compresses the air in the top half. When the pressure reaches the cut off point in the control box the pump shuts down. However you still have between 40 to 50 pounds of pressure pushing the water out to your system. As the pressure drops in the tank when you make a draw the pump kicks on and everything starts Your air setting on the tank should be 2 pounds UNDER the cut in point in the pressure control box. If your pressure control has a 20 PSI cut in and 40 PSI cut out the tank pressure should read 18 PSI. The bladder tank setting MUST be made with the pump off and the system bled down by opening a cold water faucet after you shut the system down at the breaker box. Hope this helps and thank you for rating my reply. Tom

That is probably the most asked water well question on this board. SB gave a great answer. I would only add this. As you use your tank for a few months, you will notice that problem lessen. There is initially some air both below and above the bladder. As the weeks go by, the air below the bladder will dissolve into the water, thus opening more space for water to fill the lower part of the tank.