Semagic is only for Windows, it's not for Mac. There are a few Mac clients, but I haven't come across one that's as fully-featured as Semagic. LiveJournal has a complete list of Mac clients. Check 'em out and pick the one that you like the best!
I don't think that any of them are close to Semagic. So I think it's just a matter of trying them out -- they're free, so you're not losing anything when you try them. I've never found one that I liked, so I just use their web interface.
Sorry, didn't mean to be unclear. On LiveJournal, you don't need to use a client to write your posts, you can do so directly on their website: post an entry to your journal.
Since I haven't found a Mac client to LJ that I like, that's all that I use.