Loaded dice are dice that are improperly balanced, so that the player who uses them has an unfair advantage. 

In the latter part of Chapter 6, Minerva gambles with the dictator Trujillo, using dice that were given to Trujillo by her Tio Chiche, a notorious cheater.  Minerva notices right away that the sides of the dice do not balance as they should, and sardonically realizes, "of course, my good-for-nothing uncle would give his buddy loaded dice".

The metaphor of the dice signifies the impossiblity of having a fair chance when going up against Trujillo.  Since both Minerva and Trujillo use the same dice when they toss, they end up in a draw, and have to "call it even, for now".  For a moment, Minerva imagines going up against Trujillo when the stakes are "evenly balanced, his will on one side, (hers) on the other", and feels that if such a situation should ever be possible, she might have a chance at beating him.  Minerva knows, however, that what she imagines will never happen.  At the playing table and in the larger scheme of politics and life, Trujillo always plays with "loaded dice" - he will always win, using whatever treachery that is necessary, and it is impossible to stand up against him.