While this is an older thread I thought I'd throw in my 2 cents to maybe clear things up.
1. You must have aspell installed on your linux box (don't know about windows). aspell is the spell checker that is needed before you can go any further.
To check if you have it installed you can run
rpm -q aspell-en (or whatever language you are using other than -en)
Of course if you're not using CentOS/Redhat rpm use whatever package manager you use to find out if you even have aspell. If you don't have it then you must get it before proceeding.
2. Good. Now you have aspell installed. Is your php install setup to use it though? The php install must have pspell compiled in. Yes, pspell, not aspell. pspell is a php module that uses aspell.
Use the old trusty phpinfo() command to find out:
<?php
phpinfo();
?>
Run phpinfo() and do a search for pspell on the output screen. If your search shows this:
'--without-pspell'
Then you don't have pspell compiled into your php install. So either live without spellcheck or recompile php yourself unless you can find a package for your distro that does have pspell setup. And as mentioned earlier many browsers have a spellcheck anyhow.
If your search shows this:
'--with-pspell'
Then you are in luck.
3. Next, while signed in to the forum as admin, click on Admin->Posts and Topics and see if spellcheck is enabled. If not click on it.
One last check you can perform is to double check your setup is to create a file which contains:
<?php
$pspell_link = pspell_new("en");
?>
And run it. If you get the error message:
Fatal error: Call to undefined function pspell_new() in /home/kfoster/spelltest.php on line 2
Then either pspell is not compiled in or something is screwy.