Plug in your MIDI keyboard. To connect to a standard desktop or laptop computer, you will need to attach a USB (or FireWire, if your computer has the necessary ports) MIDI interface to one end of your MIDI cable, using the "MIDI In" slot. Plug the opposite end of the cable into your keyboard, using the "MIDI Out" slot. Plug the USB connector into your computer's USB port.
Open your Reason software. Mac users will find it in the "Applications" folder, while Windows users will find it in the "Programs" folder or on the "Start" menu.
Open your "Preferences" window. Click "Edit" on the menu bar and click "Preferences" on the drop-down menu.
Click the "Page" menu at the top of the window and select "Keyboard and Control Surfaces" from the list of options that appears.
Click the button that reads "Auto Detect Surfaces." A list of available audio inputs will appear on your screen. Select your USB interface from the list.
Click the "MIDI Input" menu and select your interface a second time, establishing it as your primary input source for digital signals. Click "OK." Close your "Preferences" window and you can immediately begin transmitting notes from your MIDI keyboard to your Reason software.