Have you tried using the shifter in both straight gate mode and ratchet shift mode? Are you restricted to only Drive in both modes?
Maybe I'm not understanding you, but I would look for any sort of pin/bolt/etc that does not look factory, and is placed so that it restricts the movement of the shifter mechanism, and then remove it.
Here's an image I found of a stock Megashifter:
You can see what the factory metal and fasteners look like.
You may also want to familiarize yourself with the instructions if this is your first experience with a dual-mode type shifter:
http://files.dpbrands.com/bm/instructions/80690.pdf
The instructions only make reference to a blocker pin and how it affects whether the shifter is used for a 3-speed or 4-speed transmission; nothing to do with blocking access to First and Second gear.
Sorry, that's all the help I can give at this time without seeing pictures of your shifter mechanism.