Datamapper Call to undefined method Error

[eluser]WanWizard[/eluser]
If you get that error, the bootstrap isn't loaded or isn't loaded properly.

In your case $this->db points to CI's DB class, not to Datamappers.

And which version are you using exactly? Because v1.8 is quite old. I have the latest version (1.8.2.1) running on CI 2.0.2 without problems.