Have you tried using your accountid as username and the API key as password in the connect dialog?
Passing credentials in the URL using user:firstname.lastname@example.org is not very common nowdays, so I cannot even tell if it should work or not using REST.
I managed to get this working like this:
Check Force basic authentication (dont know if necessary)
Value: Basic MyAPIKEY_64bit_encoded
(The APIKEY is base64 encoded using certutil windows function from command prompt:
certutil inputFileName encodedOutputFileName,
do remember your api key must include the colon as described in the api documentation.)