Select the value of an attribute using XPath

I already have seen some answers to the similar questions, but I still can't figure out how I can get the value of the attribute 'CustomerOrderId' from the given structure using XPath expression:

<xjdf:ParameterSet Name="CustomerInfo" ProcessUsage="Customer">
    <xjdf:Parameter>
        <xjdf:Part ProductPart="PRD_DE150023880X01"/>
        <xjdf:CustomerInfo CustomerID="1439140" CustomerOrderID="DE150023880X01"/>
    </xjdf:Parameter>
</xjdf:ParameterSet>

What I tried so far are following expressions:

/*/xjdf:ParameterSet[@Name='CustomerInfo'][@ProcessUsage='Customer']/xjdf:Parameter/xjdf:CustomerInfo/@CustomerOrderID

and

//xjdf:ParameterSet[@Name='CustomerInfo'][@ProcessUsage='Customer']/xjdf:Parameter/xjdf:CustomerInfo/@CustomerOrderID

Maybe the problem is that I have a namespaced xml?