Connectify Hotspot usually displays the hostname for client machines that connect to your network.  Sometimes, however, Connectify Hotspot is unable to acquire the machine's name from it automatically (since Apple went to iOS 4 on the iPhone, this has become a frequent occurrence, unfortunately).  When Connectify Hotspot cannot find any name for a computer it will use the MAC address which is generally something hard to remember like "ab-3f-2c-5d...".

In Clients tab, you can right click the Client with the name you want to change. The menu that is displayed will contain the choice to "Rename".  Select this and enter a new name.  Connectify Hotspot will remember this name and use it any time that the device connects to your network again.