as said in plugin documentation , just put a "+" sign before the image name to import.
The reason "exclude" is checked is that default magento behaviour for base images is to avoid replicating them in the "extra" images gallery.
However, some extensions require to have base image displayed in the extra image list.
Btw, i would also advise using the latest git version of magmi , which is much more up to date and has several enhancements