By default a BlackBerry device can send SMS so the possible causes for SMS not being able to send are:
1. There is an issue with the network service. To test this place the SIM in another device and send a test SMS to someone.
2. If you open "Messages > Press Trackwheel" is there a "Compose SMS" option? If this option is missing the device may have previously been on a BlackBerry Enterprise Server and an I.T policy was set to disable this service. In this event you need someone to assign it to another BES with a Blank I.T. Policy.
In order to use a BlackBerry device with a Mac you need to either download the free
PocketMac for BlackBerry or purchase a copy of
The Missing Sync for BlackBerry.
Also if you can provider further information on exactly what happens when you try and send an SMS?