Can't find my AWS shared AMI when trying to share with specific account

I'm not sure about shared AMIs, but many things in EC2 are segmented by region and you have to select the correct region to see them. You can select the region in the upper right-hand corner of the screen.

Besides making sure you're in the right region, you might check if you're not filtering by "Owned by me" which is the default. You need to filter by "Private images" as per the following documentation:

http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/usingsharedamis-finding.html