Bubbles from the airlock are not always indicative of fermentation. The vigorous fermentation could be over with after a few days, although it probably still has some time left to finish fermenting. I would give it another 7 to 10 days and check the gravity readings until its down to fg and...
I personally use an electric scale and place a large enough bowl onto the scale, zero it out with the bowl on it, then proceed to measure out the extract by dumping it in or even scooping it in with a spoon or scoop until you reach the desired weight. Good luck!
Its probably still "green" or young, give it a few weeks to carbonate and condition. Depending upon the style and type of beer it may take a little longer to be in prime condition to drink, not that you cant try it sooner!
If you leave it for 2-3 weeks in the primary, you wont have to deal with sanitizing and transferring it....set it and forget about it for a few weeks!!!
You can easily go 3 to 4 weeks in the Mr. Beer keg without any issues...done it many times myself. Usually leaving the beer longer will result in a clearer beer overall, unless its a wheat beer! I have only done a handful of batches, and I typically primary for 3-4 weeks and have had clear...