Please assist - Error in app.config and Program.cs

I'm not sure what you are trying to accomplish with your app.config file. An app.config that provides the value you want need only contain

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <appSettings>
    <add key="DefaultTimer" value="90" />
  </appSettings>
</configuration>

You must include System.Configuration in the usings at the top of the program and and in the References.

The following is a rewrite of your program.

using System;
using System.Configuration;
using System.Reflection;
using System.Text;

namespace Timer
    {
    static class Program
        {
        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main ( )
            {

            if ( Environment.CommandLine.Contains ( "?" ) )
                {
                StringBuilder sb = new StringBuilder ( );

                sb.AppendFormat (
                    "Timer v {0} " +
                    "Command Line Options: /Timer:<timeinminutes> " +
                    "Current value is: {1}",
                    Assembly.GetEntryAssembly ( ).
                             GetName ( ).Version.ToString ( ),
                    ConfigurationManager.AppSettings [ 
                                             "DefaultTimer" ] );
                // do something with sb.ToString ( )
                }
            }
        }
    }

Some thoughts:

• Do not declare auxiliary variables (e.g., assm, assmName, ver, etc.) unless you will use them later in the application.
  
• Use the StringBuilder AppendFormat method rather than invoking the StringBuilder Append method multiple times.
  
• I do not recognize .Properties in your code.
  

Hope that helps.