(This is all from the documentation/getting started)
You don't need the following line:
routes.IgnoreRoute("elmah.axd");
The next line takes care of it.
Everything you need to configure is in your web.config
file. Something like:
<elmah>
<security allowRemoteAccess="yes" />
<errorLog type="Elmah.SqlErrorLog, Elmah" connectionStringName="mySqlConnString" />
</elmah>
<location path="elmah.axd">
<system.web>
<authorization>
<allow roles="Administrator" />
<deny users="*" />
</authorization>
</system.web>
</location>
Should get you going.