There are several ways to have phpBB2 mods installed on your SiteGround phpBB hosting account:

You can order phpBB2 mod installation as part of SiteGround exclusive Professional services at:

Customer Area -> Get Support section-> HelpDesk-> Exclusive Professional Services-> Adding modules to applications

You can use EasyMOD for installing some phpBB2 mods

EasyMOD is an automatic mod installer for phpBB and we strongly recommend that you take a look at it and see whether the mod you wish to install at your phpBB2 forum is compatible with it.

EasyMOD can be downloaded from here:
http://sourceforge.net/project/showfiles.php?group_id=136984

 
Please, find more information about it and how to use it at:
http://easymod.sourceforge.net/readme

 
You can install the phpBB2 mods manually

Every phpBB2 module has it own way to be installed. The general installation includes a manual modification of several phpBB2 files and some modification in its MySQL database. We strongly advise that you always read carefully the README and INSTALLATION instructions that come within the module file package. We also strongly recommend that you make full backup of the phpBB2 files and database prior to proceeding with the module installation.

If you have problems with the installation of phpBB mods, you may contact your web host for assistance. SiteGround provides the best phpBB hosting and we will do our best to assist you with your phpBB problems.