Not sure what you are asking, but do you mean you need to convert the datainject string to json ?
If so, attach a json node, which will parse it to an object.
The working with messages page in the documentation explains how to use the dining to get the right path to the string and you can then use that change node to move the string to being msg.payload
Do you mean you want to get the whole datainject string (which is the value of the datainject object) or do you mean you want to get the contents of the value field in that string? That is 5560 in the example you posted.
If the former then you just need to use a Change node to move the string into the payload (see the link provided earlier to see how to do that). If the latter then you can first move the string into the payload using a Change node, then feed it through a Json node, which will convert the string to a javascript object, then use another Change node to move the value field into the payload.