Workaround:

1. On your system, go to Apple Menu > System Preferences > Security & Privacy
2. If the preferences are locked, click the Lock (lower left) and enter your admin password to unlock it
3. In the General tab, set "Allow applications downloaded from" to Anywhere
4. Accept the security warning
5. Perform the SMS client installation
6. After installation is complete, return your security settings back to normal

^{Click the image to enlarge.}