The correct path for local font sources on a TeX Live distribution either
/usr/local/texlive/texmf-local/fonts/source/<package>
for a "all users" installation or, for a "personal" installation
~/texmf/fonts/source/<package>
or
~/Library/texmf/fonts/source/<package>
(for MacTeX installed TeX Live on Mac OS X).
In the first case a maketexlsr command must be issued (with appropriate privileges).
Just to be more specific, the complete path for igo.mf should be
/usr/local/texlive/texmf-local/fonts/source/igo/igo.mf
or, for the personal tree, either
~/texmf/fonts/source/igo/igo.mf
or
~/Library/texmf/fonts/source/igo/igo.mf
depending on the platform.
The prefix for the TeX Live/Debian might be slightly different for system-wide installation, but the personal tree is in the same location (~/texmf/).
Don't use the metafont subtree for source files.
put the file in ~/texmf/tex/latex/bbm/ (you may have to create directories for that). however, the fonts are distributed separately as metafont source and you'll need to install them, too; it's a long time since i've done that, but try copying all of the font files on an archive mirror into ~/texmf/fonts/bbm
after that, i reckon it’s worth running texhash ~/texmf (it will still work without, but slower).
now: do you really want to do all that? what did you want the double-struck fonts for in the first place?
Best Answer
In order to install packages in addition to the MacTeX distribution, you need to set up the directory structure yourself in that location. Alan Munn has written an App that does this for you automatically. That App is available here.
Alternatively, if you want to install the file structure yourself (or for anyone who does not have a Mac, as Alan's App only works for Mac computers), here is some documentation that explains the file structure and a summary of what the file structure should be.