The documentation on the METAR weather block as well as the PEAR Services_Weather module is not too extensive and user-friendly.
Here is how it works:
You install the PEAR Services_Weather module as root from the command line:
pear install Services_Weather
then you change to your pear directory (usually /usr/share/php) and look for the data/Services_Weather directory
It contains a file buildMetarDB.php.
I assume you have the command line PHP interpreter installed and it's called "php" (some distributions call it "php5").
php buildMetarDB.php -a -u horde_db_user -p horde_db_pass -d horde_db
Note: I haven't yet found a way to use the METAR block if you use another database than the horde database.
The horde_db_user is often called "horde" and the horde_db is also often called "horde". If I knew all your passwords, I wouldn't tell you ;)
Now login to horde, go to the portal, choose METAR block and click the edit button in the upper right of the block.
The rest is self-explanatory click-and-play.
--Ralf Lang