Cortex characteristics should be able to go down to d2
 

But the lowest you can currently select is d4.

I have a mentally deficient character I'm currently playing in Serenity, with a d2 Intelligence, but Herolab won't let me buy his Int down that far.

Use the permanent adjustments in the personal tab or the temporary adjustments in the in-play tab to knock it down a step.

Serenity, pg 32 (right-hand column, just above the die costs table): "No one (not even Jayne) can start with an attribute lower than d4."

One of the reasons to have adjustments is to enable house rules like the one you're using. You could also add a permanent adjustment to your attribute points to get back the points for d4 - d2.

Sorry, I'm talking about Cortex, not Serenity. Page 5 of the Cortex book details how characteristics, skills and traits can range from d2 to d12 at beginning. Serenity operates differently.

Core rules, pg 17 (bottom right corner); "Also, attributes cannot start at a level below d4."

Sorry I was reading page 9
 

Sorry, I was actually reading page 9, where it only says "While characters could possibly have Attributes of anything d2 or higher, d2 is a crippling deficiency, and d12 is the extremely high end of normal human potential."

Doesn't say anything about it not being allowed there, which is where I got that. :D