Took me all of 5 minutes, make this change to your aircraft.cfg file after placing the c17 flare effects into your Effects folder. As stated use the 'i' key to activate the flares.

[smokesystem]
smoke.0                  =-20, -59, -6, cs52_smoke
smoke.1                  =  0, -34, -6, cs52_smoke
smoke.2                  =  0,  34, -6, cs52_smoke
smoke.3                  =-20,  59, -6, cs52_smoke
smoke.4                  = -5.00, -4.00, -3.00, fx_c17flrL
smoke.5                  = -5.00, -4.00, 3.00, fx_c17flrR
smoke.6                  = -5.00, -4.00, -3.00, fx_c17flrL
smoke.7                  = -5.00, -4.00, 3.00, fx_c17flrR

or you can try these, places flares under the wings.

smoke.4                  = 5, -20.00, -10.00, fx_c17flrL      //Port Inner - Sprays Left
smoke.5                  = -26.00, -78.00, -10.00, fx_c17flrR      //Port outer - Sprays Right
smoke.6                  = 5, 20.00, -10.00, fx_c17flrR      //Stbd Inner - Sprays Right
smoke.7                  = -26.00, 78.00, -10.00, fx_c17flrL      //Stbd outer - Sprays Left

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