1. If you are in a 2D file, you can't do isometrics. check that your seed file is a 3D and/or that the file you've got up on your monitor is 3D. (It will say so at the upper left-hand corner of the window, something like "filename.dgn (3D - V8 DGN)").
2. Click on the the View Control button at the lower left-hand corner of your active drawing window. That's the button that looks like an x-y-z axis. If you don't have that button (it should be next to the + and - and other view control buttons) do Tools > View Conctrol.
3. The View control dialog box comes up (it's called "rotate view") and has a pull-down menu. Choose "isometric" or "right isometric" from this menu and there you are. The rest is on your own. No more help from me!