I can't believe I'm having the same issue in 2014 with 12.04.
Neither the GUI nor command-line shapeloader install using the opengeo-suite.
I simply followed the noob-friendly directions on this website:
http://www.staygeo.com/2013/05/enabling-postgis-shapefile-and-dbf.html, which parallel the instructions by RK almost 2 years ago.
"Install opengeo-postgis2-shapeloader using this command at terminal:
sudo apt-get install opengeo-postgis2-shapeloader
It gets installed in /usr/lib/postgresql/9.2/bin as shp2pgsql-gui. Now create a symbolic link for the same in /usr/bin directory. Prior to this you may want to check if the postgresql version you have is 9.2.
sudo ln -s /usr/lib/postgresql/9.2/bin/shp2pgsql-gui /usr/bin/shp2pgsql-gui
Now it's the time to edit plugins.d file. Use the following command to open the file for editing:
gksudo gedit /usr/share/pgadmin3/plugins.d/plugins.ini
Paste the following contents at the end of the file:
; pgShapeLoader (Linux):
Title=PostGIS Shapefile and DBF loader
Command=$$PGBINDIR/shp2pgsql-gui -U $$USERNAME -d $$DATABASE -p $$PORT -h $$HOSTNAME
Description=Open a PostGIS ESRI Shapefile or Plain dbf loader console to the current database.
Note: You may have to set the environment variable PGBINDIR on your machine. To check if the environment variable is already set type this on your terminal:
If an empty line appears as a result, it indicates that environment variable is not set. You will have to set it to /usr/bin (the location where the symbolic link for shapefile loader resides). For that use this command:
Once the above steps are done invoke pgAdmin III to see the 'PostGIS Shapefile and DBF Loader' listed under plugins."
No copyright intended (it's well-written so I didn't paraphrase :).