I have a level 9 Ranger and I want to dual him to a Cleric, but it doesn't work. Plus the manual suggests that I can't dual a Ranger. Is this true? I've seen discussion about Ranger/Clerics on this board so how do I dual one?

You should be able to dual to a second class if the class required stats are high enough and your alignment aligns with the classes.
Are you a true class Ranger, 'cause some kits can't do it.

A good way to check is to create a new Char, make sure the new classes stats line up, and start a game. If you can dual you'll be able to do it immediately. I've dualled as a cleric/ranger, but never the other way, so good luck.

Thanks. I'll have to check tonight on the stats and alignment. I'm starting to think a character can be dualed cleric/ranger but not ranger/cleric -- but I'm also very new to the game.

If I understand the manual correctly. If you start with a cleric (level 7) and then dual to a ranger, then the character can only use ranger skills until he is a level 7 ranger. But then what? Does the character progress in both classes after that or only one?

Say you start as a Lvl 7 cleric and then dual to a Ranger. Until you reach Lvl 8 as a Ranger you CANNOT use your clerical abilities - no spells. Once your second class surpasses your first you gain the firsts abilities back. Thereafter you can only progress in your second class.
So for you, you'd forevermore be a lvl 7 Cleric, but keep progressing as a Ranger. NOTE: Clerics can never use edged weapons, even in dual class, so if you gain proficiencies a Cleric can't use, you'll lose them when you become an active Cleric

You can dual a ranger to cleric (at least you could in Icewind). You just have to be human. The rule is only humans can dual class. Only demi-humans can multiclass. Just be sure to put his proficiencies in weapons you can use as a cleric (sling, flail, and dual wield are the best, probably, and an offhand weapon like mace need only have one proficiency since you won't get an extra attack on your offhand anyway).