coi fi'i sabrinas
When Lojbanizing place names, the basic rule is to start with the pronunciation used in the place itself. For Athens, you'd start with the local or standard modern Greek pronunciation (the latter, I think, is
/aˈθina/), and then approximating it with the Lojban sound system, adding a consonant to the end if necessary. Hence, {.atinas.}.
Good luck in your studies and feel free to ask questions here or on IRC (#lojban on the Freenode network -- lojban.org/irc/).
mu'o mi'e komfo,amonan