Yes you have done it correctly, now you can take screenshots of your PSP scren by ptessing the music note button.
the entries in the recovery menu should be like (approx if i remember correctly)
1.) screenshotsplugin v1.2.prx[vsh]
2.) screenshotsplugin v1.2.prx[game]
3.) screenshotsplugin v1.2.prx[game150]
below the cxmb plugin (ctf themes entry) and hv placedms0:/seplugins/Screenshot Plugin v1.2.prx
ms0:/seplugins/Screenshot Plugin v1.2.prx