PowerPivot let users create compelling self-service BI from a variety of data sources, including "Dallas" datasets. With "Dallas" CTP3, Microsoft removed the 100 row "Dallas" datasets import limit, also strengthened security; with CTP3 you must enter account key yourself. "You can open Datasets in PowerPivot from "Dallas" Service Explorer by clicking "Analyze". This open PowerPivot on your machine and show Table Import Wizard.
At this point, provide your "Dallas" account key. To do this, click "Advanced" button. Change "Integrated Security" property to "Basic", set "User ID" property to "accountKey", and change "Persist Security Info" to "True". You can now paste "Dallas" account key in Password property. Your account key is located on Account Key page of "Dallas" portal. Click OK, complete the import wizard. For existing connections to "Dallas" datasets to continue to refresh, you'll need to modify these properties by clicking on Existing Connections button in Design ribbon tab. Select connection and click "Edit"," explains Microsoft.