You can purchase a FlexPass by searching in LONG TERM parking (vs. hourly), whether you are searching by address or zone ID you always have the option of choosing hourly or long term.
To view FlexPasses you need to search in long term. See screenshot for reference.
Once you land on the purchase page, you can choose which FlexPass to purchase, as sometimes there will be a choice.
If there is no choice just click the one option to move forward to the checkout page.