I had similar problems phatal and ended up having to load the plugin manually or via another configuration script on each server boot. I could not get it to load properly via the server.cfg or other files.
For me I did the following
Make a new dir called 'TFTrue' under the 'tf' folder
'tf/TFTrue/'
Place the files in that folder and then via the command line for your game session or HLSW rcon you can manually load the plugin
plugin_load ../tf/TFTrue/TFTrue
Do not include the actual full file name just the path to the files and the files with no filename extension. And CaSe matters.
That should load the plugin and echo back a plugin load success and to further confirm its working typing 'tftrue_' + TAB should show the available commands. From there you will want to modify the 'tftrue_crit_multiple 2" which will tell it to only do 2 X damage instead of 3 X as default. If you want to increase the rate of crits then do 'tftrue_crit_chance 10' and that should double the chances or pretty much make every shot a crit.
If that works correctly then you just need to either do it manually on start of the game or inject the plugin_load line into the configuration file for another plugin. I use the config file for the hp_regeneration to do this and tweak a few other settings myself. Works very well for me.
# Example from my plugin.hpregeneration.cfg file
sm_telered_time 1
sm_teleblue_time 1
sm_ammopacks_small 3
sm_ammopacks_medium 5
sm_ammopacks_full 10
sm_ammopacks_keep 15
sm_medipacks_keep 15
sm_medipacks_small 3
sm_medipacks_medium 5
sm_medipacks_full 15
sm_medipacks_ubercharge 100
sm_cloaksyphon 100
tftrue_crit_chance 5
tftrue_crit_multiple 2
tftrue_melee_crit_chance 66
tftrue_respawn_max 0
# End
Note I am also using SourceMod, MetaMod, Mani-Mod on this host and numerous sourcemod plugins. There is also a new SourceMod based plugin being made but it is not 100% ready yet and only allows you to modify the chances of a crit. Not the amount of damage for each crit.
See here:
http://forums.alliedmods.net/showthread.php?t=69743
If you are trying to make your hosts as cracked out as mine and need a hand let me know and I can assist you via MSN / Yahoo / Steam Chat etc.
-Teddy