Try a report first
The easiest way to add contacts to a campaign is to see if a Report already has an Add to Campaign button. This will work on a Contact-based report.
You'll be able to select the camapign and Member Status and simply add them.
If there is no Add to Campaign button
If the report was based on donation contact roles or an another object but includes contacts, you won't have the button. Here's how to manually add those contacts.
Run the report as usual, but make sure that you have included the Contact ID in the report as the first column, as shown.
Export the report to Excel.
Be sure to select Comma Delimited .scv as the export file format.
Open the file in Excel and delete these rows from the bottom of the report. Save the file in a location you can remember.
Now, go to the Campaign in Salesforce and select Add Members - Import file from the drop down underneath the Manage Members button.
This window should pop up. Select Update & Add Campaign Members.
- Select the file you saved previously
- Click Next (leave the other options as-is)
Select Contact ID for the Record Id mapping. Don't worry about any other field in the drop down area, unless you would like to include a Segment Code. In that case, be sure it's indicated in the Excel file and you can map it below as well. Then click Next.
You can ignore this screen as it will simply set the status to the default value, which is what you want. Click Import Now!
This screen will confirm the import. Wait a few minutes and reload the campaign page to see the contacts. You should also receive an email with confirmation. Click Finish to clear the window.