TileMill Connect allows you to open your ArcGIS Mxd documents directly in MapBox's TileMill software. Your layers, renderers and symbols will be converted into a TileMill project and then your data will be streamed in. TileMill has a host of extra features that you can use to enhance your maps and MapBox offers fast and efficient map tile hosting.
For more information on this process, please read this blog post
Install Arc2Earth Sync and TileMill. Log into Sync and fill in the Google account information you used when you signed up for an account.
Once Arc2Earth Sync is running, open the conect menu by right clicking on the icon in the tasktray. Turn on TileMill Connect using the top option. Connect does not run by default, you must turn it on first. This option is saved so Connect will run automatically the next time you restart Sync.
By default, TileMill Connect runs on port 54321 (the standard PostGIS port). If you need to change this, bring up the main Sync options window and goto the MapBox account. Change the value to any valid Windows TCP port number.
Click on the Style Editor menu option. From there you can open an Mxd to convert or use the Style Galleries to convert individual symbols (marker, line, fill) that can be copied directly into TileMill.
Click on the Save as TileMill Project button, this process can take a few minutes depending on how complex your map is. In TileMill, refresh the Porjects area by clicking to another tab and going back to Projects. Open the newly created project and start editing your maps!