You are missing some steps in the middle :)
First of all, you need to declare each of the "variables" passed in startProcess() as process variables.
Then you need to map the variables you want to use into your WorkItemHandler. In order to do this, you need to configure the input parameters of the Work Item Node. What you need to do is basically to map the process variable name with the work item variable name that you want to use.
After you do this, you will be able to access the variable from within the Work Item Handler.
For further examples you can take a look at this set of examples: https://github.com/esteban-aliverti/JBPM-Samples