Hi Mustafa,
Firstly, you don't really have an array, just 20 contiguous bytes the first of which is labelled "MyArray".
DS.B is the same as RMB. (No not Chinese currency but Reserve Memory Bytes)
To load them all with $AA:
LDA $AA
LDX #20
loop: STA MyArray-1,X
DBNZX loop
To assign the value $BB to a particular array element (say the fifth one)
LDA #$BB
STA MyArray+4
or
MOV #$BB,MyArray+4
Now, the array initialise example is for HC08 and I believe it needs to be modified for the RS08 as it does not have a real index register. Going off now to read AN3266 but this might give you a start.
Regards David