Several webinar attendees have wondered why they can’t see the Impression Share columns in their AdWords accounts, and what they can do about it. This tutorial answers that question. Enjoy, and please post any additional questions to comments, below.
Google calculates impression share only at the Campaign level. That’s why you want to use just 1 ad group with just 1 exact match keyword for this kind of preliminary market size research.
Navigate to the All Online Campaigns page

You’ll see just the columns above, or if you’ve activated conversion tracking, you’ll see a few more.
Customize Columns

Click the Columns button, the select Customize columns from the drop-down area
Show Competitive metrics

Click the box for Competitive metrics to activate the three impression share metrics below it.
Save the new settings

Once the three new metrics appear in the Preview (bottom right), click Save at the bottom left to return to Campaign view.
Now you’ve got your Impression Share Metrics

As long as impression share is > 10% (which it isn’t in this example), you can safely calculate total searches by dividing impressions by the impression share (expressed as a decimal, not a percentage).