OK
In the DOS command windows where you run telemac, what is the result of the command python -V
If the message indicate that python is not recognize, try with the full path C:\opentelemac-mascaret\python27\python
Normally you should obtain the python version which should be 2.7.5 (or lower)
If this is ok, maybe you could try to run your computation with the full command line:
C:\opentelemac-mascaret\python27\python c:\opentelemac-mascaret\v6p3\scripts\python27\telemac2d.py name_of_steering_file
Hope this helps