First off, you need to create a stylesheet parameter like the following:

{http://www.datapower.com/param/config}parameterName

Assign it a value (ex: test1234).

Then, in your XSLT stylesheet, do the following:

<xsl:param name="dpconfig:parameterName" select="''"/>
<xsl:variable name="parameterName" select="$dpconfig:parameterName"/>

Now, you can use the parameterName variable within the XSLT stylesheet and that internal variable will have the specific value that you assigned through the stylesheet parameter (ex: test1234).

This is assuming you did not change the namespace for the stylesheet parameters though...

Hope this helps!