I know how to form Dm but what about the extensions? Any help appreciated.

...they aren't really extensions, but rather "bass" notes. That's Nashville "slash"-method chord notitaion, where the note after the slash is the BASS note you should play...it's up to you to select the appropriate bass string.

* for Dm/C you play a Dm chord (on D4-E1 strings) and "add" a C-note at fret 3 on string A5.
* for Dm/B you hold the Dm chord and "lower" the C note down a halfstep to B-note at fret 2 on string A5.
* for Dm/Bb you continue to hold the Dm chord on the treble strings, and lower the bass note on A5-string another halfstep down to the Bb.

...not knowing the song the chords are from, but here's a movement that might illustrate the desired "sound" (bass-to-treble strings/fret position):

X X 0 7 6 5 = Dm  <---starting or "setup" chord
X X 0 5 6 5 = Dm7  (with C note on G-string..."C" is the "7th" in Key of D)
X X 0 4 6 5 = Dm6  (with B note on G-string..."B" is the "6th" in Key of D)
X X 0 3 6 5 = Dm7+5 (with Bb note on G-string ..."Bb" is the "aug5" in Key of D)
X X 0 2 3 1 = Dm <---ending or "resolving" chord

...the open "D" string is a "pedal" note and provides bass ROOT drone note.

...hope this helps