If the mesosome is located inside the cell and it originates from the cell membrane, how does it help in the formation of the cell wall? It doesn't...

Mesosomes are inward folds of part of the cell membrane in prokaryotes, or bacteria.  The inward folds help facilitate aerobic cellular respiration, which generates energy for the prokaryote in the form of ATP, or adenosine triphosphate.  Not all prokaryotes have mesosomes.  Since prokaryotes do not have mitochondria, they must draw their energy source from another structure.  As far as the cell wall is concerned, mesosomes serve as a dividing point to establish new cell walls for the new cells before the cell divides.  The mesosome can actually increase the surface area of the cell membrane.  For DNA replication, the mesomes are site-specific areas where the DNA will align during the process of cell replication.  The DNA are circular in form, and there is no nucleus.  So mesosomes help serve as an "anchor point," for lack of a better term, for DNA to align with in the process of cell replication.