The Apache enabled modules are found in "/etc/apache2/mods-enabled" as a set of .load and .conf files. If the modules you want are in the /etc/apache2/mods-available folder but not in "/etc/apache2/mods-enabled" folder, just copy the .load and .conf files over (note that the .conf file may not exist).
If there is no file in the mods-availble folder, you will need to create a new .load file in the mods-available folder to point to a module in "/usr/lib/apache2/modules". To do this, create a .load file containing the line "LoadModule xxx /usr/lib/apache2/modules/yyy.so" where xxx is the name of the module and yyy is the file name. After creating this file, you can just copy it over to the mods-enabled folder and restart apache using "sudo /etc/init.d/apache2 restart".