You might have to rename the files for the specific plane you are trying to use the sight for.
They work like this:
A-20G=a20g
A6M2=a6m2
A6M5b=a6m5b
Ar 234=ar234
B-17G=b17g
B-26B=b26b
Bf 109E-4=109e4
Bf 109F-4=109f4
Bf 109G-10=109g10
Bf 109G-2=109g2
Bf 109G-6=109g6
Bf 110C-4b=110c4
Bf 110G-2=110g2
Boston III=boston3
C-47A=c47a
C.202=c202
C.205=c205
D3A=d3a1
F4F-4=f4f4
F4U-1=f4u1
F4U-1C=f4u1c
F4U-1D=f4u1d
F4U-4=f4u4
F6F-5=f6f5
FM2=fm2
Fw 190A-5=190a5
Fw 190A-8=190a8
Fw 190D-9=190d9
Fw 190F-8=190f8
Hurricane Mk=I hurri1
Hurricane Mk IIC =hurri2c
Hurricane Mk IID=hurri2d
Il-2 Type 3=il23
Ju 88A-4=ju88a4
Ki-61-I-KAIc=ki611c
Ki-67=ki67
La-5FN=la5fn
La-7=la7
Lancaster III=lanc3
LVTA2=lvta2
LVTA4=lvta4
M-16 MGMC=m16
M-3=m3
M-8=m8
Me 262A-1=me262
Mosquito Mk VI=mossie6
N1K2-J=n1k2
Ostwind=ostwind
P-38L=p38l
P-40B=p40b
P-40E=p40e
P-47D-11=p47d11
P-47D-25=p47d25
P-47D-30=p47d30
P-51B=p51b
P-51D=p51d
Panzer IV H=panzr4h
PT Boat=ptboat
SBD=sbd5
Seafire Mk IIc=seafire
Spitfire Mk I=spit1
Spitfire Mk V=spit5
Spitfire Mk IX=spit9
Spitfire Mk XIV=spit14
Ta 152H-1=ta152h
TBM-3=tbm3
Tempest=tempest
Typhoon=typhoon
Yak-9T=yak9t
Yak-9U=yak9u
So if you want to use that sight for, say, the P-47D-30, then you would rename both files like this:
p47d30.bmp
p47d30_alpha.bmp
or for Ta 152H-1
ta152h.bmp
ta152h_alpha.bmp
Then just stick them in your sights folder, and when you up the p-47 you'll see that sight for it.
Of course, you can always use it as default.bmp and default_alpha.bmp and see it for every plane.
Hope that made more sense...