After you have been added to the VisLab calendar, you can access it online from the ISR webmail interface (http://webmail.isr.ist.utl.pt), as shown here:

Managing the calendar from Thunderbird

This takes eight simple steps, as follows:

1. Open Thunderbird, install the "Lightning" add-on and restart Thunderbird:
   
2. In the browser, log in to the ISR webmail (http://webmail.isr.ist.utl.pt) and open the calendar tab on the left. On the right side, find the "i" image on the calendar you want to use ("[alex] Vislab") and click on it. Copy the first URL from the window that will pop-up:
   
3. Back to Thunderbird: open the calendar in the top right corner. Right click and add "New Calendar...":
   
4. Choose "On the Network" option:
   
5. Paste the link that you've previously copied from webmail to the "Location:" field:
   
6. Log in using your webmail credentials, finish:
   
7. Now you can open the calendar from the top right corner button and select the calendar you want to enable/disable from the right menu:
   
8. (Optional) If you prefer a sidebar type layout, you can enable it on the bottom right corner:
   

Instructions provided by Ricardo Nunes