1) Install (via FTP) the piCal directory (with all files and subdirectories) under the "modules" directory of XOOPS.

2) Then, when you display the list of the subdirectories of "modules" you will see all the XOOPS modules, including piCal

...
newbb
news
piCal
sections
system
...

3) Login as a XOOPS webmaster

4) Click on "Administration Menu" from your "User Menu"

5) Click on the "System Admin" icon

6) Click on "Modules"

7) As you can see, the piCal module appears at the bottom of your module's list, ready to be installed.

8) Click on the button install

9) piCal is now intalled, you can start to work with it (settings, publish events, etc)