Unlike other D&D games it appears that paladins and rangers have 6 levels of spells. To cast these high level spell levels, do you have to have a wisdom of 16? Should I therefore add points to wisdom when I have the chance?

Yes, you need a wisdom of 16 to cast 6th level paladin or ranger spells.

Keep in mind that you will only be able to cast 4th level spells by the end of the normal campaign and that there are wisdom increasing items such as the Every God Ring (+5) and the Blue Ioun Stone (+2), for example, which you can use to bolster your character's wisdom.

Also keep in mind that:
- level 5 and 6 Paladin and Ranger spells are pitiful compared to level 5 and 6 Cleric and Sorc spells like Raise Dead, Heal, Cone of Cold, Mass Haste, etc.
- You won't even get your single 6th level spell until level 29, which is the end of HoF (unless you squatted/muled). If you were foolish enough to raise WIS to 22, you'll get a second 6th level spell/day.
- Not counting the front-loaded goodies they get in their first couple levels, Rangers and Paladins are basically Fighters with a handful of marginally useful spells instead of feats and Weapon Specialization.

Thanks for the reply. But another question. Let me clear this up, you get more experience if you increase the difficulty like in IWD 1?

No, not exactly.
By playing normal mode you end up around lvl 18 or so!
HOF-mode is supposed to be played after you've beaten the game with that same party. In HOF-mode all monsters are MUCH tougher to (over)compensate for your much higher lvl!
There is also much better items that costs lots of cash.

Ok, Im having a problem. My paladin can cast spells but the icon to access them is not there. I cant cast any spells because I cant select them.

What is wrong?

Thanks in advance.

You need to rearrange the toolbar. Right-click, select what you like.