1. create a folder where you would like to store attachments. Many people make this folder called "attachments" in their root OST dir.
You must make sure that this folder is writable by your webuser. (i.e. apache, iUSR_servername, etc. depending on webserver)
2. log into OST as an admin and go to Admin Panel -> Settings -> Attachments.
In the "Attachment Folder" field put the whole path to where you put the "attachments" folder.
The full path can and does vary greatly from install to install and host to host. I seem to recall that you need to use the absolute path, not a web URL.
If for some reason you do not know this path, you can create a test.php file in your attachment folder and put the following in it:
<?php
echo realpath(dirname(__FILE__));
?>
Then you can copy and paste the output into the settings in OST. Make sure that you delete this file when you are done with it.
