nowebm, when installed from ports, does not include the noweb.sty in
a location LaTeX can find it. As such, the default configuration has to be
changed for the texmf tree layout, or the texmf.cnf file edited to enable
$HOME/texmf and copy the noweb.sty file into there.
Install where LaTeX can find it by default with the ports install.
How-To-Repeat: Install nowebm. noweave foo.nw > foo.tex ; latex foo.tex
Over to maintainer of devel/noweb, which this port concerns.
Fixed in the latest version of the port.