During the installation of LOVD, all available modules are installed automatically. Only if you have uninstalled a module or upgraded your LOVD installation with an update that included a new module, you'll need to install it before you can use it.
To have LOVD scan for modules added after installation of LOVD, click the "Scan LOVD install directory for new modules" link from the Setup area. You will see a warning, explaining that you need to know that the modules are from a trusted source. Basically: use your common sense here, just like you would when you run a program on your computer, downloaded from the web. Don't just include a module in LOVD if you're not familiar with it's source.
After clicking "Next >>", LOVD will search the modules directory for modules that are not already installed. LOVD will inspect the new modules and query them to verify if they respond properly. If everything is found to be OK, LOVD will report which modules have been installed.
Please note that newly installed modules are not yet enabled.