Leveling first row of CMUs on uneven slab
Starting at the highest corner or spot, and level from there.

It can be done a few ways, first you could set the first 2" block far enough from the high side so that it is level with a thin setting bed under it, and fill the rest of the area that is too thin, with masonry cement.

Or you could set the 2" block at the highest side, using enough mud to run it level, until there is enough drop to add 4" block to keep it level.

Another way is to figure out the amount needed to cut off of each block, so a relatively uniform level/layer of mud is under each block. This way may look the best if there is no other surface to be applied to the block to hide the build up.