Ok, it looks like the bet box doesn't show a class name, so you will have to copy the contents of the bet box to the clipboard, add a value to it, and then paste the new value back into the bet box.

Near the top of your script, add the following. You will need to add in the co-ordinates of the call and raise buttons, and the bet box.

Code:
SetFormat, Float, 0.2              ;Set to 2 decimal places

BB=0.04                            ;BB size
SB=0.02                            ;SB size

BT=0                               ;Set bet total to zero

FBX=188                            ;Fold button position X
FBY=365                            ;Fold button position Y

CBX=                               ;Check/call button position X
CBY=                               ;Check/call button position Y

RBX=                               ;Raise button position X
RBY=                               ;Raise button position Y

BetBoxX=                           ;Bet box position X
BetBoxY=                           ;Bet box position Y
Then, for your hotkeys, add the following. Wheel Up should then increase the bet by one BB, and Wheel Down should decrease the bet by one SB. Then just set up another hotkey for the raise button, as you did previously for the fold button. Please note that the value that gets copied to the clipboard will be from the active window. I'm not sure how Ongame queues the tables, so you may have to add some other lines of code to get it working 100% how you want it to.

Code:
Esc::
MouseGetPos,,,win
ControlClick, x%FBX% y%FBY%, ahk_id %win%
return


WheelUp::
Click, %BetBoxX%, %BetBoxY%
Send {HOME}+{END}^c
BT:= clipboard
BT+=BB
Send {HOME}+{END}{Del}%BT%
return


WheelDown::
Click, %BetBoxX%, %BetBoxY%
Send {HOME}+{END}^c
BT:= clipboard
BT-=SB
Send {HOME}+{END}{Del}%BT%
return
Let me know how you get on.