Well that really depends on what you're trying to pull in. The easiest thing is just going to be to look at the existing tabs' logic and emulate that.
For example the 'Images' tab uses the 'LookupProductImage' XSLT function to pull up the current product's image:
Code:
<xsl:value-of select="aspdnsf:LookupProductImage(ProductID, ImageFilenameOverride, SKU, 'medium', 1, $AltText)" disable-output-escaping="yes"/>
...whereas the 'Overview' tab first checks to see if the product has more than one variant, then uses a template (defined further down in the file) to display more info:
Code:
<xsl:choose>
<xsl:when test="$isMultiVariant = true()">
<xsl:call-template name="OverviewMultiVariant"/>
</xsl:when>
<xsl:otherwise>
<xsl:call-template name="OverviewSingleVariant"/>
</xsl:otherwise>
</xsl:choose>
In the second example there, you'd need to look in the templates further down to see how things are done. You can put the logic that is in those templates directly within a div for your tab if desired, it's just easier to organize them and do 'ifs' with templates.
If you're just looking to pull in a value that you've got stored on that product, you can use something like this to get it:
Code:
select="aspdnsf:GetMLValue(Nameofthefieldyouwant)"