You can download a version of the Open Source Office Suite OpenOffice 3 with Myanmar support from the links at my.openoffice.org. This now includes support for Myanmar word based line breaking. If you install the Myanmar dictionary then it also has some spell checking support. As of OpenOffice 3.1 the main version of OpenOffice includes a Myanmar locale with Myanmar sorting support.
If you would like to help with translating (localizing) OpenOffice into Myanmar, then please have a look at the Pootle site.
On Windows you can type using Keyman. On Linux, you must configure SCIM to use XIM “On the spot” mode (in the Front End / X Window Setup options) for KMFL to work correctly with OpenOffice.