To load data into the designer from an external resource, you should do the following. Change the URL Prefix in Edit -> Preferences. This will allow you to use relative URLs within your url configuration when setting up your stores.
We could make it so that any configuration within the component config named "url" used the Url Prefix. This would allow us to load images for an image component very easily.
We are working on making the Store loading process a lot easier. For example if you could see if a request was made, to what url and what the response was. This process of debugging what's wrong with the configuration would be simplified.
I'm using Ext Designer, xds-126.96.36.199c on a Mac w/ Snow Leopard and running into the same problem. When I attempt to load a store from my local site, I immediately get an error saying "Unable to load data using the supplied configuration" yet the same store works when it's deployed. In addition, when using Designer, I see no attempt to connect to my server in the server logs. I also tried using http://www.extjs.com/forum/topics-remote.php for the store url and immediately get the same error though I can see the returned data when using this url in Firefox. Is there a configuration setting somewhere that I need to change? My apologies if I've posted to the wrong thread.
Would you be able to attach your .xds file for a quick scan? You can email it to me if you don't want to post on the forums: jarred at extjs.com
We're going to build an auto-store configuration feature, so stores will try to be intelligent and automatically configure themselves based on returned data. That'll definitely be a big help, no doubt.