To give the reviewers access to the abstracts to be reviewed, you need to send out the login credentials. This can be done from the Review Module, either in bulk or per reviewer.
To send out the login details to all reviewers at once, go to the 'People' list in the Reviewer Module. Click the "Send Unsent Login Email(s)" button.
A popup will appear:
After clicking "Yes", all reviewers will receive the following e-mail (with their own unique login details):
For an individual reviewer to receive the login details, select the reviewer in the 'People' list. Click the "Resend Reviewer Mail" button.
Now the reviewers can login, to score the abstracts they're assigned to.