The Jackal Crossbow was made on old non-steam Postal 2 version and sometimes it may not work properly.

If you have already a M4A1 Transformer mod, you do not need change the "L" key,
because it compatible with him.

If "L" doesn't work for you, you can try to do one (or more) these things:

1) Try exit game and run Postal 2 and Jackal Mod again.

2) Change arrow types manually - First open a cheat console (press " ~ ") and write "ToggleSpawnLight". Then, if you want charge next arrow, you can open console again, press ArrowUp key (that instant write last cheat into the console), and press ENTER.

3) Add allias manually (lvl hacker)- First open the System folder in the SteamApps/common/Postal2Complete/ and find User.ini file. Then find these two lines:

Aliases[X]=(Command="",Alias="None")
and
L=ToggleInvHints

when "X" are the next "empty" allias (32 are for example), and change them to:

Aliases[32]=(Command="ToggleSpawnLight",Alias="JackalCrossbow")
and
L=JackalCrossbow

4) More simply, than ad.3) is run the game, and write in the cheat console that command:
"set input L ToggleSpawnLight"
That thing automatically attach "L" key with changing next arrow command.