XPath: Select parent nodes which have a subnode with an attribute
"/Database/Package[/SystemCompatibility/System[@mtm='2055']]"

<?xml version="1.0" encoding="UTF-8"?>
<Database version="300">
<Package id="6imb05ww" description="ThinkPad Modem Adapter">
    <SystemCompatibility>
        <System mtm="8742" os="Windows XP" oslang="en" />
        <System mtm="2055" os="Windows XP" oslang="jp" />
    </SystemCompatibility>
</Package>
<Package id="6imb06ww" description="ThinkPad Modem Adapter">
    <SystemCompatibility>
        <System mtm="3046" os="Windows XP" oslang="en" />
    </SystemCompatibility>
</Package>
</Database>

/Database/Package[SystemCompatibility/System[@mtm='2055']]

/Database/Package/descendant::System[@mtm='2055']