Sametime 7.5 have new option called policies by default. So you need to set the option for the first time.
Open the STpoliies.nsf, edit the default policy and go to
ID 1001 Name Allow user to create instant meetting.
put the Type = 2
Save the document and restart your sametime server.
This will enable user to have instant meeting. Similarly there other few option that need to ned enable based on your requirement.
Check and let me know does this resolve yur problem.