The quick and dirty way to create an isometric grid in AI:
1. Draw a vertical line where you want to start the left side of the iso grid.
2. Draw another vertical line, this time where you want the right side of the iso grid to end.
3. Select both lines and click on the Blend Tool.
4. With the Blend Tool, click on the first vertical line, then the second one.
5. Double click the blend tool to bring up the blend options dialog and select "Specified Steps" or "Specified Distance", whichever suit your needs. Here specify the distance you want between the vertical lines. Click OK.
6. Select the vertical blend; Double click the Rotation Tool to bring up the Rotate dialog box.
7. In the Rotate Dialog box, specify 30 for the angle and click 'Copy'.
8. Repeat steps 6-7, but this time specify -30 for the angle.
9. Select all the 3 blends (vertical, 30, -30) then select Object > Blend > Expand.
10. Now select View > Guides > Make Guides
There's your iso grid.