In the advanced editor there is a button for HIGHLIGHT tags and it prompts you for an option. If you posting VB6 code, the option should be "vb". Use "vb.net" for VB.NET code. You can also use "sql" and others. E.g. this:

[highlight=vb.net]Dim str As String = "Hello World"[/highlight]

produces this:
vb.net Code:
  1. Dim str As String = "Hello World"
Just note that the HIGHLIGHT tag doesn't support other formatting internally. That means that you must use [code] tags if you want to use bold, underline or colouring on specific parts of your code.