How do I model the growth of investments over time?

Within the Account forms in the Plan UI, you can choose whether you want each account to grow at your plan-level investment growth / dividend rates (i.e. the ones defined in Plan Settings > Growth Rates), or at different custom rates per account.

Also note that in Plan Settings > Growth Rates, you don’t necessarily have to assume fixed rates either. If desired, you could choose a specific historical returns sequence, or define a custom sequence of returns using the Advanced editor.

And then separately in Monte Carlo mode (Chance of Success tab), returns/dividends in each trial there are controlled by whatever MC configuration you choose, e.g. historical data or custom probability distributions.