Mods can (in theory) be downloaded and used directly
as the ZIP file.

The install procedure in readme.txt is slightly dated,
'viewer' actually does the following:

* look for environment variable "OGTA_DATA"; if it exists
  this is used as the base-data source.
  By default this is the file 'gtadata.zip' searched in
  the current directory (at viewer runtime).
  
  If this source is not valid the directory where viewer
  is installed is checked before giving up.

* look for environment variable "OGTA_MOD"; if it exists
  and is a valid data-source all files contained within
  will overlay files in the base data source.
  This is not used by default.

To load a mod I use something like this:
OGTA_MOD=mods/a_mod.zip ./viewer [...] -m map_filename -g style_filename

In this case you have to use -m & -g and use the filenames
contained in the zip (note upper/lower case).

Similiar to gtadata.zip the files have to be in the 'root' of
the ZIP file, not in sub-directory.