Why can't I export .blend files to unity?

I have unity 4.5.4 and blender 2.72 an I modeled a tree in blender saved it in object mode in the cycles renderer; however, when I tried exporting to unity it said that blender cannot convert the .blend file to a .fbx file. Help?

I just tried exporting a cube from Blender 2.72 to Unity 4.6.0b20 and I can confirm that works, so the problem should be either in your blender file or your Unity version.

With 2.72 Blender’s FBX importer/exporter was updated. Sometimes there are features mismatches between different softwares that break functionality; maybe you’re using a feature Unity can’t import yet? That exact issue happened to me when using a brand new version of Blender before.
The safest solution for me was to use an older version of Blender until the next Unity update, where the issue was solved.

Double-check you’re following all the steps:
http://docs.unity3d.com/Manual/HOWTO-ImportObjectBlender.html