korell: To use the keymapper, load up the game, then once loaded press Ctrl+F1 to bring up the keymapper interface.
Now, the ALT key is the strafe modifier, so on the onscreen keyboard click on the ALT key (this is the action for which you want to create a bind).
With the ALT button selected, click the add button and then press the left cursor button on your actual keyboard. Click on add again and this time press the right cursor button. Then click on save and exit to return to the game.
Now your left and right cursor keys activate two actions, that of the strafe modifier and their own left/right action, meaning that they strafe left and right respectively.
You could make the WASD buttons do both the cursor keys and strafe modifier, but then you'll run into a problem when typing in your name and savegame names, so I just use the cursor keys for movement.
If you still have problems, here is the contents of my mapper.txt file (you can specify the keymapper filename to use in the .conf file).
hand_shutdown "key 290 mod1"
hand_capmouse "key 291 mod1"
hand_fullscr "key 13 mod2"
hand_pause "key 19 mod2"
hand_mapper "key 282 mod1"
hand_speedlock "key 293 mod2"
hand_recwave "key 287 mod1"
hand_caprawmidi "key 289 mod1 mod2"
hand_scrshot "key 286 mod1"
hand_video "key 286 mod1 mod2"
hand_decfskip "key 288 mod1"
hand_incfskip "key 289 mod1"
hand_cycledown "key 292 mod1"
hand_cycleup "key 293 mod1"
hand_caprawopl "key 288 mod1 mod2"
hand_swapimg "key 285 mod1"
key_esc "key 27"
key_f1 "key 282"
key_f2 "key 283"
key_f3 "key 284"
key_f4 "key 285"
key_f5 "key 286"
key_f6 "key 287"
key_f7 "key 288"
key_f8 "key 289"
key_f9 "key 290"
key_f10 "key 291"
key_f11 "key 292"
key_f12 "key 293"
key_grave "key 96"
key_1 "key 49"
key_2 "key 50"
key_3 "key 51"
key_4 "key 52"
key_5 "key 53"
key_6 "key 54"
key_7 "key 55"
key_8 "key 56"
key_9 "key 57"
key_0 "key 48"
key_minus "key 45"
key_equals "key 61"
key_bspace "key 8"
key_tab "key 9"
key_q "key 113"
key_w "key 119"
key_e "key 101"
key_r "key 114"
key_t "key 116"
key_y "key 121"
key_u "key 117"
key_i "key 105"
key_o "key 111"
key_p "key 112"
key_lbracket "key 91"
key_rbracket "key 93"
key_enter "key 13"
key_capslock "key 301"
key_a "key 97"
key_s "key 115"
key_d "key 100"
key_f "key 102"
key_g "key 103"
key_h "key 104"
key_j "key 106"
key_k "key 107"
key_l "key 108"
key_semicolon "key 59"
key_quote "key 39"
key_backslash "key 92"
key_lshift "key 304"
key_lessthan "key 60"
key_z "key 122"
key_x "key 120"
key_c "key 99"
key_v "key 118"
key_b "key 98"
key_n "key 110"
key_m "key 109"
key_comma "key 44"
key_period "key 46"
key_slash "key 47"
key_rshift "key 303"
key_lctrl "key 306"
key_lalt "key 275" "key 276" "key 308"
key_space "key 32"
key_ralt "key 307"
key_rctrl "key 305"
key_printscreen "key 316"
key_scrolllock "key 302"
key_pause "key 19"
key_insert "key 277"
key_home "key 278"
key_pageup "key 280"
key_delete "key 127"
key_end "key 279"
key_pagedown "key 281"
key_up "key 273"
key_left "key 276"
key_down "key 274"
key_right "key 275"
key_numlock "key 300"
key_kp_divide "key 267"
key_kp_multiply "key 268"
key_kp_minus "key 269"
key_kp_7 "key 263"
key_kp_8 "key 264"
key_kp_9 "key 265"
key_kp_plus "key 270"
key_kp_4 "key 260"
key_kp_5 "key 261"
key_kp_6 "key 262"
key_kp_1 "key 257"
key_kp_2 "key 258"
key_kp_3 "key 259"
key_kp_enter "key 271"
key_kp_0 "key 256"
key_kp_period "key 266"
jbutton_0_0
jbutton_0_1
jaxis_0_1-
jaxis_0_1+
jaxis_0_0-
jaxis_0_0+
jbutton_0_2
jbutton_0_3
jbutton_1_0
jbutton_1_1
jaxis_0_2-
jaxis_0_2+
jaxis_0_3-
jaxis_0_3+
jaxis_1_0-
jaxis_1_0+
jaxis_1_1-
jaxis_1_1+
jbutton_0_4
jbutton_0_5
jhat_0_0_0
jhat_0_0_3
jhat_0_0_2
jhat_0_0_1
mod_1 "key 306" "key 305"
mod_2 "key 308" "key 307"
mod_3