Changes to the XML file for custom parameter types
From Joomla! Documentation
You need to make two specific amendments to the XML configuration file. First, you need to ensure that the file is included in the <files> list:
<files> ........ <filename>elements/index.html</filename> <filename>elements/newparm.php</filename> ........ </files>
Tip: It is good practice to also include an empty index.html file in the elements directory as an extra security precaution to prevent directory listings being returned by the web server.
Alternatively, you can simply include the entire elements directory using a <folders> element:
<files> ........ <folder>elements</folder> ........ </files>
The second amendment that you need to make to the XML configuration file is to let the Joomla Framework know that you have added a new parameter type and where to find the code to support it. To do this you add an addpath argument to the <params> tag:
<params addpath=”[path]/elements”> ...... list of <param> elements ...... </params>
Notice that you are adding a path to a directory containing the parameter type code, not to the code file itself. Notice also that you only have one addpath argument so if you are adding multiple parameter types you need to gather them into the same sub-directory and that sub-directory must contain only parameter type definition code.