I know how to do this but with blender 2.49b only. I t seems it doesn't work for newer versions of blender.
1 download the mesh exporter:
BlenderExport.zip and unzip the file.
2 copy
ogremeshesexporter.py, ogrehelp, and ogrepkg and paste where blender is installed at
...\blender\.blender\scripts, just like the next image:

3 download and install python 2.6:
for x86 computers:
python-2.6.msifor x86-64 computers:
python-2.6.amd64.msi4 now open blender and export your model.
click file->export->ogre meshes (if all was correctly installed this menu will exist)
Select the path where the mesh will be exported, then click preferences button.

Select manual path for the xmlconverter, and put the path of the converter. and click OK.

click export and you have now a mesh file.
blender 2.49b link is here if you don't have it:
blender-2.49b-windows.exeNote: you must use Ogre3D version 1.6 command line tools, NOT 1.7. download it at:
OgreCommandLineTools_1.6.3.msi