How to I show pop-up reminders when TSS XP first opens?

You can configure TSS to show pop-up reminders for daily notes when the program is first opened.

First go to the schedule screen and add a new daily note.

Under the details of the daily note, check the 'Reminder' box to trigger TSS to show a popup reminder on startup, the reminder will show if the current date is within the date range the daily note is set for.

Close and restart TSS to see the popup reminder.