You need to have an account with Adobe technical support in order to download the API documentation, although a description of the API can be found in the product documentation which is publicly available.
The product documentation for Adobe Campaign v6 can be found here: Documentation 6.1
The product documentation for Adobe Campaign Standard can be found here: https://docs.campaign.adobe.com/
To request a support account, send an email to: email@example.com
To login to the support web site for Adobe Campaign go to: Adobe Campaign extranet - Login Page
Go to the Download Center to get the jsapi chm file.