They only recently rolled out impression share metrics, and it's not always easy to figure out how to get them.

In order to get adcenter impression share numbers in your reports:

You must run one of the following reports:

Account Performance
Campaign Performance
Ad Group Performance

and where you choose what unit of time to view, you must select Day.

If all of the above conditions are met for your report, then you will be able to use the choose columns feature to include some new columns:

Impression Share
Impression share lost to budget
impression share lost to rank
impression shar lost to relevance
impression share lost to other.

If you aren't running the right report or the day view, these column choices either won't show up or will be grayed out.

Hope this makes somebody's life easier, it took me a while to figure out.