There is no way to choose input/output variables of the element type for the message activities in case wsdl messages
with 1 part.
Bpel specifiaction allows to use input/output Variables of the correspondent xsd element type in case wsdl message has
only 1 part.

From the WS-BPEL 2.0 spec: 
"
[SA00048] When the optional inputVariable and outputVariable attributes are being used in
an <invoke> activity, the variables referenced by inputVariable and outputVariable MUST
be messageType variables whose QName matches the QName of the input and output message
type used in the operation, respectively, except as follows: if the WSDL operation used in an
<invoke> activity uses a message containing exactly one part which itself is defined using an
element, then a variable of the same element type as used to define the part MAY be referenced
by the inputVariable and outputVariable attributes respectively."

But variable chooser doesn't allow to choose variables of the element type.

The sample project is in attachment.