You have to shift the lever to the most "relaxed" position, that is to the smallest cog. Put some tension on the cable by hand and shift the inner lever at least seven times. You should feel the cable lengthen a little each time until you are in the high gear position. Then the cable head should be visable from the outside of the lever just as it was for the front brifter and you just push it straight out. You, of course, have to pull the brake lever inward (as if applying the brakes) to see the cable head.
If this doesn't work, there is something wrong with the brifter.