Failed to load plug-in: 'immersion.dle'

[NI_LAB]

Hi all,
I have just this one line in my code:

immersion = viz.add('immersion.dle')

but I get this error

************************************************** ****************************
** Loading Vizard2.py
************************************************** ****************************
** ERROR: Failed to load plug-in: 'immersion.dle'
** Load Time: 0.00 seconds

If I try to do this:
vrpn = viz.add('vrpn7.dle')
everything works fine, and both files are in the same folder.

What am I doing wrong?