You know, I had no clue what you were talking about when you first posted this, but last night, I had the same problem, only with Poser 5. Actually, I didn't even get that menu - after naming the file and hitting "save," it looked like it did it, but it didn't. I finally had to switch to ProPack to export my morph target.

I think Poser's OBJ export is rather buggy. Sometimes, you can't "as morph target" on that menu, but clicking on the Windows taskbar "adjust date and time" fixes it. The only thing I can suggest is reinstalling. Poser tends to get corrupt easily.

Also, if you tried removing the "mac" i/o plugin to speed up your library, try putting it back. I loved the way that sped up my library, but I couldn't export OBJs properly without it.