If you'd like the donor to pay via credit card you can do that by adding via the Reports > All Pending (same as adding auction items). To add the donation by cash, check, or other methods:
1.
Search for the donor by their email or name. You can also create a new donor.
2.
Select the campaign under the pulldown.
3.
Add the amount.
4.
Select the donation date.
5.
Select the donation type.
6.
How the donation was paid.
7.
Add an optional memo. This is only seen by the organization.