How can I add my transactions?

You can add transactions in multiple ways:

from any page, using the search bar to find the stock
in the favorites list page, by clicking on the three dots to the left of any favorite to access its action menu
in the details page of any favorite by clicking on the add transaction button
from the transactions page by clicking on the add transaction button
from the transactions page by importing a CSV or an Interactive Brokers statement
automatically, by connecting your broker in Settings → Brokers

For a non-time-consuming setup, instead of adding all your historical transactions, you could start off by adding your current average cost basis and selecting today's date as a transaction date.