In ODOO 16, to assign an existing employee to an accrual plan, you should navigate to the "Employees" menu, select the employee you want to assign, and then edit their record. In the "Leave" tab, you should be able to select the appropriate accrual plan from a drop-down menu.

Regarding the accrual plans, yes, their purpose is to allocate leave (such as vacation days) to employees automatically based on a set schedule, such as a certain number of days per year or per pay period.

Regarding your issue that the accrual plans do not seem to be working, there could be a few possible reasons for this. One possibility is that the accrual rules within the plan are not configured correctly. Another possibility is that there is an issue with the schedule on which the plan is set to run. Additionally, there could be a bug in the software or a conflict with another module. You may want to check the plan configuration, schedule and check if there any error message or log. 

1. Activate developer mode
2. On Odoo landing page, start typing Scheduled Actions to search for the menu.
3. Go to Scheduled Actions and search for Accrual.
4. Open it and Run Manually. Also ensure that it's active for future accruals.
   

This should add the accrued hours/days. Hopefully this one will be set to run easier in the future.