How did you got the certificate? You should generate the private key and certificate together. You can use the "Private Key and Certificate Generator" in our plugin to generate these. After that you need to upload the certificate to the Rabobank iDEAL dashboard. You als need to fill in the "Private Key Password" on the iDEAL dashboard.
In the following topic you may find some more information about the configuration of the iDEAL Advanced variant:
We've tried to make it easier, but the iDEAL Advanced variant is quite complex. If you don't have any experience with iDEAL and security certificates you can ofcourse outsource the configuration: