I also was not able to install ps using PEAR. Here are the steps I took to getting it installed:
1. you may need to install the perl module XML::Parser (http://search.cpan.org/~msergeant/XML-Parser/Parser.pm) to get pslib to install
2. install pslib (http://pslib.sourceforge.net/)
3. install ps (http://pecl.php.net/package/ps) using the instructions from (http://www.php.net/manual/en/install.pecl.phpize.php)
4. copy the ps.so file created by to the php extensions directory (not automatically done)
5. edit php.ini to include the ps extension (not automatically done)
If you are running FreeBSD, this link may help you with problems installing pslib (http://www.freebsd.org/cgi/query-pr.cgi?pr=84406).
Note that pslib (the library) and ps (the php extension) are not the same.