The filemaker documentation is pretty clear on how to set up the server and file you want to share via ODBC from with in Filemaker. However it did take some trial and error to set up the connection with the ODBC admin in OS X. Its been a while since i did this but I'll give you what i remember.
You need to place the ODBC driver and ini files in the system Library/ODBC/ (I think i had to create the ODBC folder). Then set the Owner to "System" with Read/write, Group "Admin" read only. This applies for all files with in the ODBC folder.
Then open the ODBC admin, create a new driver. Give it a description, i used "Sequelink ODBC". Driver file "/Library/ODBC/SequeLink.bundle/Contents/MacOS/ivslk18.dylib". Set up file i left empty. Define as "System". select OK.
You then create a System DSN. When it asks what driver type "Sequelink ODBC". You ll then need to name that Data Source... and give it a description. You ll need to configure the following five item for the driver.
User Value (user name in FM that you want to access the database)
Password Value (for the above user)
Host Value (the ip address of the FM server box)
Port 2399
Server/DataSource Value ( the table name of the data you want to access)
Then press OK and you should be good to go.