If you are a regular Salesforce user, when you log in to your Salesforce org you are automatically logged in to PSA.
If you are a Timecard Only or Expense Only user, log in to your secure website. Use your email address as the user ID and enter the PIN that you have been provided with.
If you are a vendor or partner, log in through the Salesforce Partner Portal
Allows partner users to log in to Salesforce through a Web portal rather than through salesforce.com..
See the Salesforce Help for more information.