Reminders are supported only in the standard folders. This restriction of Outlook^® applies also to Public ShareFolder. A Public Folder of an Exchange^® server knows also no reminders.

http://support.microsoft.com/?kbid=186689

Also from a subfolder of your personal calendar you wouldn´t get no reminder. What you could do is to synchronise the Public Calendar with your personal standard calendar with the Public SyncTool. Than you will get also a reminder, but you will get it from your personal standard calendar.