Howto: Enable Debuging in SharePoint Web Application
English :
I know that people are already blogging this topic, but I just want to make summary of the procedure.
1. You need to modify 3 lines in web.config
2. The 3 lines are :
a. <SafeMode MaxControls="200" CallStack="true" DirectFileDependencies="10" TotalFileDependencies="50" AllowPageLevelTrace="false">
b. <customErrors mode="Off" />
c. <compilation batch="true" debug="true" >
Alternatively you can install “Debugger Feature” from codeplex. (http://www.codeplex.com/features)