First, please check whether the option has been enabled under:
Overview => Settings => Main Settings for the Review Process
The option is called "Priority Topic Selection". It is enabled by default (see image 1).
Second, please go this page:
Overview => Settings => Conference Phases and Deadlines
... to activate the module and set the phase in the section "Reviewers Can Select Their Priority Topics" (see Image 2).
As soon as the option has been enabled, the reviewers can select their priority topics (see Image 3)