It should be the report server web service URL that you connect to (ex: http://dwh:80/reportserver). In order to use that you need to confirm the following:

- The reporting services web service has been configured for that instance of SSRS
- You have the correct URL for the web service. You should be able to get this from the admin managing your SSRS instance
- The account that you are using has rights to upload reports to the location you are trying to upload to

I am guessing you are using a stand-alone version of Report Builder? Another thing you could try would be to get with the admin of the SSRS instance and see if the Report Builder click-once app is configured. If so you could open Report Builder from within your SSRS instance and it would automatically be connected to the right URL.

What do you get when you try and connect now?