Version: 4.2.7.2
Build ID: 420m0(Build:2)
You tagged your quetsion ‘Conversion’. If this is actually your intention you should select the part of text to be converted and subsequently go > ‘Insert’ > ‘Object’ > ‘Formula’. If nothing is selected then Math will start with an empty formula this way.
V 4.2.7 may contain a few extra bugs. The recent V 4.3.5 might do better.
Did you have already a look into the Writer and Math guide? If so were exactly do you have the problem?
The free guides can be downloaded from http://www.libreoffice.org/get-help/documentation/