I've written a procedure for configuring Reaper to use a Donner DMK25 mini MIDI keyboard as a control surface.
Donner DMK25 MIDI keyboard setup for Reaper
1 Note that upon powerup (connecting via USB), the top 4 pads will blink in succession, left to right, 4 times at increasing speed. This is normal.
2 In Reaper, go to Options, Preferences (bottom of the list, or just press “p”).
3 Under Audio, select MIDI Devices, and add DMK25.
4 Right-click it, and select “Enable input from this device” and “Enable input for control messages”. Click OK.
5 If DMK25 appears under MIDI output, make sure it is disabled (i.e. both of those Enable boxes are unchecked). Click OK.
6 If you don’t have a soundcard or audio interface, select “Microsoft GS Wavetable Synth” and “Enable output to this device”. Click OK.
7 Under Preferences, General, select Keyboard/Multitouch. Click on “Assign keyboard shortcuts to actions or change existing shortcuts”.
8 The Actions window opens. In the Filter: box, type “transport” (without the quotes). All 6 of the DMK25’s transport buttons can be connected with their desired functions in this section.
9 Scroll down to “Transport Fast forward a little bit”, and click Add (in the Shortcuts… box).
10 The Shortcut box should be highlighted and say “<<type key or move controller>>”. Press the Fast Forward button [>>] on the DMK25, then click OK.
11 Scroll down to “Transport Play” (or “Transport Play/pause” if you prefer), and click Add. Press the Play key [>] on the DMK25, then click OK.
12 Scroll down to “Transport Record” [O], click Add, press the Record key, and click OK.
13 Scroll down to “Transport Rewind a little bit”, click Add, press the Rewind key [<<], click OK.
14 Scroll down to “Transport Stop”, click Add, press the Stop key [ ], click OK.
15 Scroll down to “Transport Toggle repeat”, press Add, press the Repeat key [<~>], click OK.
16 Click Close in the Actions window. Click OK in Reaper Preferences.
17 This last step changes the Record button from a momentary function to a toggle function. All thanks to Demonic Sweaters on Youtube for this! Start watching at 4:58 into the video here:
https://www.youtube.com/watch?v=LSzrbA9WQmk
And now you can control your Reaper transport functions from your Donner DMK25 MIDI keyboard.