Hi Ian - welcome to SilverStripe.
Installing modules is pretty easy once you know how to do it - but as with all things until you know how then it appears complicated.
The steps are:
- download the module and place it in the root directory of your site (alongside cms, sapphire, mysite, assets etc..)
- then visit www.yoursite.com/dev/build, this will run a script that will update the database with all the details needed for the new module
- some module also require you to add bits to your mysite/_config.php file but these should say in the Readme filel
I realise there isn't a 'dev' folder in your sites directory but in this case it is recognised as a special command by SilverStripe to run the script. This works in a similar way as each page that you create in your site doesn't actually create a 'contact-us.html' page in your sites folder.
Until you have run /dev/build accessing the admin will cause an error as it it trying to load the modules code but the database doesn't contain everything it needs.
Does this make sense? Let us know if you need any other help.