Hi,
What version of Windows are you running?
I currently have this set-up with the same version of Sybase Central and it has the 15.7 plug-in on a Windows 7 box. So check the following:
Double check the permission on the sql.ini file.
When you added the ASE did you use dsedit or the menu option Sybase - Connectivity - Open Client Directory Service Editor? Can you ping the server from this tool?
It could be if you moved the sql.ini file from somewhere else it got spaces put in it so check the file for unwanted characters.
Sql.ini uses client to some extent it could be the version C++ redistributable package you have installed on your machine also, but you would see a side-by-side error or something to that extent.
What version of the C++ Redistributable package are you running on your Windows machine.
Thanks,
Dawn Kim