All you have to do is select the field you want to display as link and then change the code from:

<xsl:value-of select="@VariableName"></xsl:value-of>

to

<xsl:value-of select="@VariableName" display-output-escaping="yes"></xsl:value-of>

This works in SharePoint 2010, if you’re on 2013 you should use the following (notice the closing tag):

<xsl:value-of select="@AssignedTo" disable-output-escaping="yes" />

Thanks to Ryan Tate for pointing it out.

+507