You haven’t given any information about your system specs so this may not apply to your system if it is a mac.
If using CB 7.5 on a windows computer:
I would suggest to go out and purchase a midi cable that goes from midi in/out to a USB interface. Then all you need to do is to plug in the cable to the midi in & out on the back of your keyboard to a USB port on the PC. Make sure the cable is plugged in (and windows installed the driver) before starting CB as CB will not detect the midi cable interface if it is plugged in after CB is already started. The device will show up in Cubase under devices>device setup>midi>midi port setup. Then all you need to do is turn on the keyboard and activate the midi function. Again, you did not mention what kind system or keyboard you have but usually on the old keyboards you will need to somehow activate the midi function. This may be a midi button that you need to press on the keyboard or maybe a combination of buttons you will need to press on the keyboard to activate midi. Hopefully you still have the manual for the keyboard. If not, happy searching the internet for it.
Once you get it activated just create an “instrument track” and make sure that the “Input Routing” for that track is set to “All midi Inputs”. Hopefully at this point the keyboard will work and you can adjust it and assign sounds through CB.
Again, if that USB cable somehow gets disconnected while Cubase is running you will have to restart CB for it to re-detect it. That function has been requested numerous times in our CB forum but so far we don’t have it (unless to are using a mac).
Hope this helps a bit and I wish you good luck and that you have fun with your new (old) CB 7.5 
Also, don’t forget to check out if your CB 7.5 is upgradable for free which is available if the 7.5 was purchased during the allowed grace period.
Regards 
