Where's the C terminal on my boiler control?
Unfortunately, the Aquastat you're using does not expose a C terminal. The transformer is likely soldered directly to the board; so unless you're good with a solder gun, you're going to have to buy a different thermostat.
The red wire attached to the T terminal, should be connected to the R terminal of the thermostat. The white wire attached to the T terminal, should be connected to the W terminal of the thermostat. When the thermostat calls for heat, it will connect the R terminal to the W terminal. This will cause the coil to pull a switch closed (relay), which tells the boiler that the thermostat is cold.
If you're not afraid to potentially break things, and you really want to try to get the new thermostat working. You could try to add your own C wire. You'll simply have to connect the new C wire, to the other lead of the secondary coil of the transformer.

Left image shows original wiring. Right image shows where the C terminal should be added.
WARNING:
Modifying the wiring of the Aquastat will likely void the warranty, and could potentially damage the equipment. Proceed at your own risk.