This is either because the tag group you wish to send an email campaign out to has not been set up as a global tag - inside Settings, Global Settings, but instead has been set up as an adhoc tag.

Please make sure that any tag groups you wish to send out an email campaign to have been set up as global tags, as it is only global tags that are displayed on the Send Campaign window - as shown below:

If however you have set your tag up as a global tag but you do not see the tag word listed (as shown above) it could also be that the contacts in this tag group do not have email addresses.

The tag groups will only be displayed if contacts applied with the tag(s) have email addresses.

Please check your contacts in the relevant tag group to make sure they have email addresses against their contact records.