You want to add pod 'CloudXUnityAdsAdapter', '3.3.0-beta' similar to the following to your Podfile:

target 'MyApp' do
  pod 'CloudXUnityAdsAdapter', '3.3.0-beta'
end

Then run a pod install inside your terminal, or from CocoaPods.app.

Alternatively to give it a test run, run the command:

pod try CloudXUnityAdsAdapter