Hello Jayne!
Q: 1. If I use Mass Product Changer all descriptions become the same.
A: The Mass Product Changer addon applies the value you have set to the selected products. And if you have specified the certain description value, the addon will apply it.
Q: 2. If I export descriptions the HTML is in lines not columns and I am unable to import when changed.
A: You should try to use the Open Office Spread Sheet tool to manage the .csv files.
You can download the Open Office tools here:
http://www.openoffice.org/
Also please make sure that you have selected the correct separators in Import/Export Wizard:
http://screencast.com/t/u1WdSfhpZ