In order to develop new client-side web parts using the SharePoint Framework (SPFx) preview, you will need the following correctly setup:
If you try to browse to your site (i.e. https://mytenant.sharepoint.com/sites/dev/Shared%20Documents/workbench.aspx) and miss or forget step #5 , you will get the following error:
'ClientSideApplicationId' does not exists in list or list item
All of these instructions can be found here.
I'm a SharePoint Server MVP and Solution Architect who enjoys working with the most cutting-edge Microsoft technologies.