Adding words to Vocabulary

Not exactly a bug, but not sure where else to post this. When I go through lesson materials, I often see new words in questions in the "Expansion" section. For instance - Newbie - Stopping the Taxi lesson, has a verb "to walk" in the Expansion set of questions (but doesn't have this word in the "Vocabulary" section. I want to be able to add it to the vocabulary tool so that I can then revise it through flashcards. Anyway of doing this?

UPDATE: Please Ignore the message, I have just noticed a big sign "left click to save words"! apologies

Thank you

Javad