To find your Dwolla Master Account Id, you will need to call the root of the API.
Sandbox - GET https://api-sandbox.dwolla.com/
Production - GET https://api.dwolla.com/ 

Example Request and Response - 

GET https://api-sandbox.dwolla.com/
Accept: application/vnd.dwolla.v1.hal+json
Authorization: Bearer pBA9fVDBEyYZCEsLf/wKehyh1RTpzjUj5KzIRfDi0wKTii7DqY
{
  "_links": {
    "account": {
      "href": "https://api-sandbox.dwolla.com/accounts/ad5f2162-404a-4c4c-994e-6ab6c3a13254"
    },
    "customers": {
      "href": "https://api-sandbox.dwolla.com/customers"
    }
  }
}
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