You should have uploaded the directory 'admin' to your webspace.
Open your internet browser an go to the url of the admin page.
It should be something like this:
http://www.yoursite.com/admin/
Login with the password, which you have set in the password.php file in the admin directory.
On the right side you see your directories. When you click on them, you can open them. When you open a directory with pictures, this pictures will be loaded on the left side (list). You can click on one of this pictures. You will see a thumbnail of the picture on the left side. On the bottom you see a white box. Here you can type a description. When ready, save it with the 'save data' button.
An example of admin is here:
http://imagevuex.inn.no/imagevue/admin/