Installing mayaseed is simple; simply move the downloaded files to your preferred location on your disk or network and open the file included with the mayaseed distribution.

Once you have installed mayaseed, you just need to give maya a restart and enable it from the plug-in manager Window > Settings/Preferences > Plug-in manager

Mayaseed in the plug-in manager

During export, mayaseed exports geometry as obj files. Mayaseed includes two .obj exporters, one cross-platform and cross-version exporter built using pyhton and one faster compiled version that is specific to your OS and maya version. If you have a version of maya with a compatable compiled exporter you'll want to enable that also; it will be called ms_export_obj_<maya_version>.

If something goes wrong or you prefer to install mayaseed manually, create a file called mayaseed.module with the following contents, replacing <version_number> and <install_directory> with the correct values.

mayaseed <version_number> <install_directory>
icons: graphics
scripts: scripts

You then want to put that file in one of the following locations:

Mac: /Users/<username\>/Library/Preferences/Autodesk/maya/<maya version>/scripts
Windows Vista and higher: C:\Users\<username>\Documents\maya\<maya version>\scripts (you may have My Documents instead of Documents)
Windows XP and lower: C:\Documents and Settings\username\My Documents\maya\<maya version>\scripts
Linux: /usr/aw/userconfig/maya/<maya version>/scripts