can anybody please explain, how i can assign the 8 rotary knobs on the nektar lx 49+ in my Cubase5 , so I can control the 4 frequency bands and their settings?
Welcome ! 
As I doubt that there was a specific Nektar LX49+ remote device included in Cubase 5, you’ll probably need to use a Generic Remote Definition, for this. In Cubase 11, it looks like this (actually, it’s still basically the same as it was in Cubase 5, even if a redesign of the window happened, since) :
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To add a Generic Remote definition, you should go in the Studio Setup window (if I remember well, it was accessible with the Devices > Devices setup command in C5), and add a Remote device with the + button on the upper left of it.
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Set the MIDI ports used.
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Add each knob of your LX49+ in the upper panel, using eventually the Learn option
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be sure that each knob has the relevent column values (“controller”, MIDI channel and address used by the involved knob, the maximal value it uses and the R and P flags (read + pick-up).
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Proceed in the same way in the lower panel, by adding again the same knobs defined in the upper one.
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Define the command that each knob will control : to control each of the EQ strip 4 bands gain and frequency, I set them as shown above (EQ1 Gain, EQ1 Freq, EQ2 gain, EQ2 Freq and so on…), but in Cubase 5 it was probably be labelled differently. It works, here, at least…
Once properly set, do not forget to save the GR definition created, just in case…