It appears that you need to tell log4net to use the Trace appender, which, according to the documentation, writes the events using the System.Diagnostics.Trace.Write(string,string) method.
Here is how to change the log4net section in app.config or web.config (if you keep yor log4net configuration in this file) - note this (LOGGER) in the pattern, it is there to distinguish the output from log4net from any other messages in Output window:
<log4net> <root> <level value="DEBUG" /> <appender-ref ref="MyTraceAppender"/> </root> <appender name="MyTraceAppender" type="log4net.Appender.TraceAppender"> <layout type="log4net.Layout.PatternLayout"> <conversionPattern value="(LOGGER) %-5p %d [%t] %c %m%n"/> </layout> </appender> </log4net>