What keyboard did you connect the MD-BT01 to, we cannot help you without at least that information...

Yamaha CVP-107

Hello

Did you find anything ?

In pairing the BD-BT01 with your MAC (this step doesn't really depend on the keyboard - although good to know for end-to-end info) - your Mac is older and its Bluetooth "radio" does not support BTLE while the BD-BT01 uses BTLE. In order to add BTLE support to Macs without this support embedded, the easiest way is to add a BTLE USB dongle.

Some info for another product is here:

http://www.macworld.co.uk/how-to/mac-software/get-continuity-handoff-airdrop-on-old-mac-3582632/

Arguably the simplest and cheapest way forward is to use an inexpensive Bluetooth 4.0 USB dongle. [... We] have tested three USB dongles (Asus BT400, IOGEAR GBU521 and GMYLE), and our advice is to use one of them – in our tests we purchased the GMYLE dongle for around £10 from Amazon.

Note that the dongles that are said to work in the article are because the software programmed to interface with BTLE was patched to work with these. I would not use this specific information as anything definitive in your case - but only as general guidance that a BTLE dongle (USB device) may bridge the gap between old hardware and newer protocols.

My advice would be to get a reputable manufacturer (Asus seems fine) dongle from a local store with a liberal return policy if you have problems with the solution.

Upgrading the Airport card/device is said to be difficult - but something to look into for your specific hardware as an alternative.

Upgrading your entire machine to post 2014 would assure BTLE is in the machine (you can see a chart somewhere for hardware specifics as for different models BTLE was introduced earlier). This is "expensive" - but an option.

Connecting the MD-BT01 does require a connection to a 5-pin MIDI equipped keyboard... your original post only mentions a computer and the Bluetooth device. Contact Apple about new computer hardware capable of providing a solution; Bluetooth LE was just being introduced in 2010, so they may have a hardware recommendation for your model of iMac.