Steps to add an xslt file to your style library using a feature in SP2010:
1.> In VS2010 create a new Element;
2.> Under the element add and xslt, change the "Deployment Type" property;
3.> Move the elements to "Items in the Feature" default is "Items in the Solution";
4.> Modify the elements.xml file;

5.> Ensure the itemStyleCustom.xsl exists in the Xsl Style library.
