Presently, i don't have it installed, so, i can only give you brief overview but it is very easy to use
Once you open the application,
1. Choose the type of transmission line (for you microstrip).
2. Then enter the charactersitic impedance
3. You must also enter the dielectric properties of the material you are using.
4. After entering all the above and pressing calculate gives you length and width...
I have donwloaded a snap shot of the APP CAD window and attached it here....