I don't do mr beer but I use priming sugar(dextrose) for all my beers.
The proper procedure for beer batches is the following:
Calculate amount of sugar required based on batch size, temp and style
Boil1 cup water, add sugar, dissolve, cool to 70
Add to bottling bucket and rack in beer to self mix as it swirls in
Bottle using a wand
The mr beer process may be different I don't know but I saw you got no replies so I figured I'd jump in and try to help