In CRM 4 OnPremise it was pretty easy to turn on Dev errors so that you (CRM Admin types) could get a detailed error message when something went wrong rather than a friendly error message that said Contact your CRM Admin. This was not so helpful when you were the admin seeing the error.
All one had to do in CRM 4 OnPremise was to edit the Web.config file in the Inetpub\wwwrooot director for the CRM site and change the DevErrors key to On.
For CRM 2011 OnPremise it is done the same way as CRM 4.
What’s one to do in CRM 2011 Online? At this time, there is not any method for enabling Dev Errors via the Settings Panel or any other administrative mechanism. The way you have to do it is to call CRM support (1.877.276.2464), open up a ticket, and ask them to turn on development errors. There is not any charge for this request but it does take a day or so for it to occur depending on how busy life is at Microsoft CRM Support.
I am wishing that in the next UR release that they add this feature to the Settings Panel so that a CRM Admin can switch it on and off as necessary.
If you agree that this would be a good feature for the next release, go out to the Microsoft Connect site and vote for my suggestion. You will need to login with your Windows Live ID first to enable the link, then register for Microsoft Dynamics CRM Feedback and then the link will be accessible (whew!, but it is worth it).