Search your computer for Refills. If installed correctly, you will locate them either in the same folder with your Reason application or in your Library folder under Reason/Refills. If you find them elsewhere on your computer, move them to one of those two folders so Reason can access them.
Open Reason by double-clicking on it in your Application or Programs folder. If you get an error message about sound output, click "OK" or "Cancel." Plug in a set of external speakers or headphones.
Add a new instrument. Click on "Add Device" in the top menu bar, and click on one of the Instrument buttons (not a mixer). When it opens, hit "Tab" to view the back of the instrument and make sure the "Output" cords are attached to one of the "Input" slots in the mixer that opens automatically in each project. If these cords aren't attached, you won't hear any sound.
Choose the Refill you want. Next to the name of the current instrument, you will see arrows that let you scroll through the various Refill instruments for this Instrument style. If you want a different Refill type, click the file icon to the left of the instrument name. It will automatically take you to the Refills folder. Different instrument types will have different file endings. If you choose one with a different file ending, the interface will change when you return to the mixer screen.
Try out the Refill instrument by playing on the onscreen keyboard, playing the A to L and Q to O lines on your keyboard or playing an attached USB MIDI keyboard. Keep trying different Refill instruments until you find the one you want.