Check if identity for glance was created under keystone and appropriate credentials were entered in glance-api.conf and glance-registry.conf:

admin_tenant_name=
admin_user=
admin_password=

Make sure glance-api-paste.ini & glance-registry-paste.ini has following entry:

[filter:authtoken]
paste.filter_factory = keystoneclient.middleware.auth_token:filter_factory
delay_auth_decision = true

Make sure glance-registry-paste.ini has following entry:

[pipeline:glance-registry-keystone]
pipeline = authtoken context registryapp

Restart both glance-api & glance-registry services. Now you should be able to access glance service from horizon.