How to resolve this error related to exacttarget apis?

I'm trying to integrate with the https://auth.exacttargetapis.com/v1/requestToken endpoint to receive an AuthToken to communicate with the Salesforce REST API, like here.
I created a Connected App and received my Consumer Key and Consumer Secret (clientId & clientSecret) but I keep receiving the following error when I try to post my credentials to the endpoint:
{ "message": "Unauthorised", "errorcode": 1, "documentation": "" }

That is the crux of my issue which could be a number of different things, but here is some other pertinent information:

• The User I created the Connected App with, my own user, is a "System Administrator" with "API Enabled" checked

• I am on a free version, I have not paid for any licence

• I have selected all possible scopes for the Connected App under "Selected OAuth Scopes"

• I allowed "all users to self-authorise" as via this answer

• I am unable to find the "API Integration Component" as described here under the Package Manager / Installed Packages fields, which seems to be a point of importance in all the docs that I have seen.