I have figured it out from other resources...
Below is the working code. Hopefully, Someone else will find it useful. :)
#============================================================================
# DOS command:
#
# powershell -NoExit -nonInteractive c:\ps1\001\LoadDLL.ps1
#
#============================================================================
[void][System.Reflection.Assembly]::LoadFile("C:\ps1\001\IS2008DllTest.dll");
$objTest = new-object IS2008DllTest.Test;
$objTest.Initialize();