I had the same problems with QGIS 2.6.1 and LAStools - "global name 'os' is not defined..."
I tried many things and helped me following:
Although the lastoolbox is already installed in 2.6.1, download newest version from this address: http:\\
lastools.org\download\lastools.zip
Extract this file and copy folder "LAStools" somwhere. Path to this folder must not contain gaps, diacritic.........example: c:\q gis\LAStools........is wrong, c:\q_gis\LAStools.......is ok :)
Extract file "QGIS_2_4_toolbox.zip" from "LAStools\QGIS_toolbox" ...........the name of the folder will be "processing" and replace with original folder in your system.............example: C:\QGIS_Brighton\apps\qgis\python\plugins\processing
Set the path to "LAStools" folder in QGIS>processing>option and configuration>tools for LiDAR data>LAStools folder
Hope it will help...
Tomas