Why 'Add what you're doing or feeling' option/icon is missing from my Android Facebook app? How should I enable it?
Facebook released v2.3 around 20th March 2013, Here's a snapshot of the page with Facebookv2.3.  

Facebook announced it's enhanced status update on how they're feeling around 10th April 2013 which is the reason it's not included in the App.  

Update the app, you'll get this feature.