The answer will depend on the dll. If it's Managed (.Net) then you can simply call:
System.Reflection.Assembly.LoadFile("ddlFile");
And then use reflection to call the method. (you may need to instance a class via reflection as well).
If it's Native (like c/c++) then you'll need to register the function via a DLLImport.
http://msdn.microsoft.com/en-us/library/ms235282.aspx[
^]