If you are using a Mac, you may need to check an additional setting.

1.     Open FrontlineSMS from the Apps folder, from the menu, click File, Get Info.

2.     In the General tab, select the Open in 32-bit mode checkbox.

3.     Restart the process of taking your modem out, turning FrontlineSMS and the modem software off, then turning the modem back on to allow the modem software to register the device and starting FrontlineSMS.

Another user tip from the forum that has proved useful for some Mac users is the following:

If it appears that there is a problem with permissions on the “/var/lock” directory it may help to change the permissions so that you can read and write to this directory. More specifically, use Finder to navigate to “/var/” in the root. Then, if you select the “/var/lock/” folder, right click, and select “Get Info”. In the lower right, there will be a small padlock icon that’s most likely in the locked position. Click it (and enter your password). Change the permissions (at the bottom), until it says that you can both read and write to the folder.