What does it mean to "print money"? In my intro macroeconomics course, my professor tried to emphasize that the money supply changes ONLY by increasing or decreasing lending activity. However, I feel like he was trying to focus on lending because most people have the misconception that the Federal Reserve just "prints money" to increase the money supply. However, is it true that although the MAJORITY of the money supply increases with more lending, that in some cases the Federal Reserve does "add zeros" to their spreadsheets to influence the money supply? For example, if they wanted to increase the money supply they could "print money" and use those funds to buy bonds from banks which in turn would then lend out that money and increase the money supply.
Depends on what you mean by money supply: M0, M1 etc (see here for example). If M0, then printing money is literally issuing new coins and bills. 

The Fed can certainly "add zeroes" and use the extra money to buy other assets. The frequency of this is an empirical question. 

Ask your professor what she/he meant.