Dominion randomizer deck confusion
Don't use them to randomize.  Use one of the online card pickers:

What is the Best Random Card Setup Tool?

Use them as the last card in their respective stacks.  They are used to indicate that the stack has run out so that it is easier to know how close the game is to ending.  @krisa suggests that if you use sleeves to use a different colored sleeve for the randomizer cards.  This will make it much easier to keep them separate.

Pg. 5 of the rules says:


  Players may also use the Randomizer cards
  as Placeholders to mark the card piles so
  empty piles are easily seen.