I have to insert serial number and reg code every time I launch ChemDraw in MacOS X.
The serial number is stored in /Library/Preferences/Network/com.cambridgesoft.plist. The only way you could be seeing this problem is if you didn't have privileges to write to that file. Download the MacOS X Repair Privileges Utility discussed at http://docs.info.apple.com/article.html?artnum=106900
to correct the problem.