I can only monitor by asking Marketo support? Is there a better way to self serve? Either in Salesforce or Marketo?
Replace the app-XX## with whatever your instance's pod subdomain is (when you log into your Marketo instance, it's the first part of the URL in your browser)
I actually migrated away from that tool since Marketo now has a sync status tool built in that was released Q1 of this year. If you navigate to Admin > Integration > Salesforce, there is a tab not for Sync Status. Screenshot below, but that's where you can get all of your sync stats at a better level of detail and clarity than the legacy tool. However, I did just check and the legacy tool still loads and runs just fine for me.
One thing I will say, since the UI update you can't look in the URL string anymore for your datacenter. Instead, go to Admin > My Account, and there is a box on that page with Support Information. Your datacenter will be listed there. Just plug that code into the URL in place of DATACENTER: https://app-DATACENTER.marketo.com/supportTools/sfdcSyncStats