In order to use fastboot, you're going to need the ASUS ADB/Fastboot drivers found here. Even though it says it's for the 201, they are good for any TF model and I use them for my TF700.
ASUSTeK Computer Inc. -Support- Drivers and Download Eee Pad Transformer Prime TF201
The problem is that they aren't in exe form and have to be installed manually through the device manager. Download the zip file, and unzip them to an easy to find place. Then open up the device manager, and plug the tablet into the computer in fastboot mode. It should pop up as an unknown device, probably as Android 1.0 or something like that. Right click it and click update software. Navigate to the place you unziped the drivers to and click ok. It should install the drivers.
After the drivers are installed, you can install the Android sdk provided earlier. You will want to know where it installs to because you will have to navigate to that place to use the fastboot command.
In the command prompt, use the command "cd C:/ " except type the actual path as to where the fastboot file is located in the sdk. Make sure the TWRP file is in the same folder, and then use the command "fastboot flash recovery 'twrp.img'" or what ever the file name for twrp is. after that you should be able to boot into twrp and flash the rom from the sd card.