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...