Audacity’s equalizer can only be used after the tape has been captured (recorded).
Try playing an mp3 recording you already have in Audacity to experiment with the equalizer, (it shouldn’t be greyed out then).
Note: unlike the equalizer on your stereo Audacity’s equalizer cannot be adjusted in real-time.
http://audacityteam.org/help/faq?s=recording&i=records-tapes