The cloudapi is generally the same URL as the vCloud portal / user interface. In vCloud Director 9 there are 2 URLs (one for the legacy / Flash UI and one for the new one).
For example, if your vCloud Organisation is called 'acmecorp' and your cloud provider is at https://provider.cloud.com/ then:
vCloud Flash (legacy) UI: https://provider.cloud.com/cloud/org/acmecorp/
vCloud New (HTML5) UI: https://provider.cloud.com/tenant/acmecorp/
API endpoint: https://provider.cloud.com/
URL for branding logo (for example): https://provider.cloud.com/cloudapi/branding/logo
Before you can retrieve or make changes to the API, you need to authenticate and obtain a x-vcloud-authorization token and then use that token in subsequent requests - William Lam has a good series explaining this (from both a web browser and using the command-line tool curl) at: Exploring the vCloud REST API Part 1 - VMware vSphere Blog
Hope that helps.
