hi,
in my host there is ASP.NET WEB APPLICATIONS tab what provides ddn3.1 installing. for installing ddn we should pass following steps:
1- select the ddn3.1 installation button 
2- define name of subdirectory
3- define the name of database
4- the installation process finishes
the problem is in here that when i want to run ddn it show a error like below:

Runtime Error

Description: An application error occurred on the server. The current custom error settings for this application prevent the details of the application error from being viewed remotely (for security reasons). It could, however, be viewed by browsers running on the local server machine.

Details: To enable the details of this specific error message to be viewable on remote machines, please create a <customErrors> tag within a "web.config" configuration file located in the root directory of the current web application. This <customErrors> tag should then have its "mode" attribute set to "Off".

<!-- Web.Config Configuration File -->

<configuration>
    <system.web>
        <customErrors mode="Off"/>
    </system.web>
</configuration>

<!-- Web.Config Configuration File -->

<configuration>
    <system.web>
        <customErrors mode="RemoteOnly" defaultRedirect="mycustompage.htm"/>
    </system.web>
</configuration>

Now i want some one guide me and say what my mistake is and also why should i install it in a subdirectory , why not in the root. I really appraciate it.