I just ran into this as well and since there doesn't appear to have been a answer here I ended up finding out the solution myself. PHP is luckily already compiled with the pear option, but the pear script doesn't exist.
1. The pear script is just a script wrapper to the php binary so all you need to do is create the script. Here is a typical pear script from a PHP 4.x compile so just create /usr/local/psa/apache/bin/pear with the following content which includes the paths relevant to Plesk's install on FreeBSD(make sure it is executable):
--------------------------------------------------------------------
#!/bin/sh
# first find which PHP binary to use
if test "x$PHP_PEAR_PHP_BIN" != "x"; then
PHP="$PHP_PEAR_PHP_BIN"
else
if test "/usr/local/psa/apache/bin/php" = '@'php_bin'@'; then
PHP=php
else
PHP="/usr/local/psa/apache/bin/php"
fi
fi
# then look for the right pear include dir
if test "x$PHP_PEAR_INSTALL_DIR" != "x"; then
INCDIR=$PHP_PEAR_INSTALL_DIR
INCARG="-d include_path=$PHP_PEAR_INSTALL_DIR"
else
if test "/usr/local/psa/apache/share/pear" = '@'php_dir'@'; then
INCDIR=`dirname $0`
INCARG=""
else
INCDIR="/usr/local/psa/apache/share/pear"
INCARG="-d include_path=/usr/local/psa/apache/share/pear"
fi
fi
exec $PHP -C -q $INCARG -d output_buffering=1 $INCDIR/pearcmd.php "$@"
--------------------------------------------------------------------
2. Now create the directory in the script, which is the default PEAR directory defined by Plesk(/usr/local/psa/apache/share/pear).
3. Lastly, copy over the existing PEAR tree in /usr/local/psa/psa-horde/pear from horde into /usr/local/psa/apache/share/pear so you have a copy of the needed pearcmd.php in there as well as the versions of base modules.
That should be it. I've only installed new modules so be careful if you plan to upgrade any as it may have some negative effects on horde, although I wouldn't think so. Back everything up first!
--
Jared