Right now, the only way to manually add donations is through the Offline payment gateway. So to add this donation, go to Charitable > Settings > Payment Gateways and temporarily enable the Offline donation gateway.
After doing that, go to your website and create a donation for the amount, choosing the Offline payment option.
Once you have made the donation, you can turn off the Offline gateway if you don’t want to use it, and you can go to Charitable > Donations to mark the donation as Paid (by default, offline donations are considered “Pending” until they are confirmed as paid).
This is definitely something we want to improve in the future, probably by adding an extension to be able to add donations manually from the dashboard.