Hello,
You will need to apply the latest device pack to your CUCM (cmterm-devicepack8.6.2.21020-1.cop.sgn) released 28 FEB 2012.
This adds support for the MX300 and SX20 as well as they are new products. For some reason this is not mentioned in the device pack release notes but apply this and then you should have support for your MX300.
Please read the device pack release notes for how to install.
Thanks