Where do I enter income during retirement?

That depends: are you trying to model earning additional income during retirement years? Or is it that you prefer to think about distributions from investment accounts as income and would like to model a specific amount of those in certain years?

If the former, you should be able to simply create a Custom income stream that starts/stops at the specific milestones or years you desire.

But if the latter, something to note is that PL computes drawdown/withdrawals automatically based on everything in your plan, e.g. income, expenses, asset purchases/sales, taxes, etc.

So in most cases, drawdown amounts are not something you would define manually. Instead, you might model an increase in living expenses (or other expenses) during retirement years, and withdrawals occur as-needed, according to the sequence defined in Plan Settings > Drawdown.

Another mode you might enjoy playing around with a bit is Withdrawal Strategy mode, but keep in mind that WS mode ignores most of your normal planned items when it goes into effect, so that you can explore what one of the “textbook” withdrawal strategies would look like instead, starting in a specific year.