Options under "Effect" grayed out

I am trying to censor some parts of a song following this guide, http://www.instructables.com/id/How-To-Censor-A-Song/step8/Mute-The-Part-You-Wish-To-Censor/, but when I get to the linked step it won’t let me use the “amplify” option – it is grayed out along with everything else except for “Hard Limiter” and “SC4”. What am I doing wrong?

Press the yellow Stop button: http://manual.audacityteam.org/o/man/faq_editing.html#grayed.