For some reason all the Apache docs tell you to run apxs over a .so file. I never had any .so files, but I found this to work (if you have the .c file for your module).

/usr/local/apache2/bin/apxs -c mod_foo.c
/usr/local/apache2/bin/apxs -i -a -n foo mod_foo.la

It seems that the apxs compilation process puts the .so file in a folder called .libs/ which is invisible to a regular ls, but the above works too.