How are Salesforce Campaigns sorted when importing via the import wizard?

When you begin importing data into Campaign Monitor via the import wizard, you'll be prompted to select a Salesforce Campaign. To help you find the most relevant campaigns quickly, they are sorted by their 'Last Modified Date,' placing newer or recently modified campaigns at the top of the list. For performance and usability, only the first 500 campaigns are displayed.

if you can't find a campaign easily consider making a small change and saving it so the last modified date is updated pushing the campaign to the top of the list.