I would not try and "upgrade" from 1.0.x to 1.5. It's a total rewrite of the platform. You need to migrate. The database structure has changed. It's like 2 forks, now.
Info on how to here:
I am surprised that Dreamhost has just flipped to 1.5 without keeping a 1.0.14 version for all their current Joomla users. Switching from one version to another is not trivial. Many current components and modules will not work in 1.5, possibly ever - even with Legacy Mode on.
You could use the DH installer to create your new 1.5 installation, however, and then migrate from your existing installation to the new one and future upgrades - through the one-click - should work.