The Business Central token connects DocuBuilder with Business Central, so DocuBuilder can receive information.
Step by Step
Open Business Central and search for Users.
Open the appropriate user.
In case the Web key already exists, copy it (by 3-cbl-click) and paste it in the field Web key in DocuBuilder.
In case you create a new Web Service Access Key, choose to NOT let this access key expire after a certain period (since DocuBuilder stops working when a Access Key expires).