The only way to access your various site is by going to your main partner site's admin and logging in. You should be presented a dropdown list of your various sites to select from. If not sure what your partner site is provide any site url that you are working with and I'll help verify.
Another option is to access your partner portal and from here you can go to the "clients" tab and select whichever site you need to access. - https://securesyd.worldsecuresystems.com/PartnerPortal/Index.aspx
Except the dropdown only appears the first time you log in. If I then log back in it takes me to the same site without offering the dropdown. I'm sure I could clear history and cookies and get the dropdown again, but is that really the only way. Should be a pretty basic function.