Do you have mala set to only show working roms?
If so, search for the entry in your mame.xml file and change the last part to:
driver status="good" emulation="good" color="good" sound="good" graphic="good"
Instead, I went into Mala options and turned off that feature. Now SMW shows up. Thank you.
BadMouth's suggestion is a good one. That way you don't have to have all the extra 'Non working roms' showing up in yoiur list
Weell...I don't have any non-working roms.
Indeed, But the XML file does. ;-) It has all games working or not
And as the xml file isn't alphabetized, looking through it quickly annoyed me.
Try doing a search for a word.

anyway......You can always choose to filter all of the non working games out via MaLa.
But you think you will loose a game 'Super Mario World' .....Maybe this is just an error in the XML file.?
So the suggestion is rather than having all of Non working games in your game list (that you don't have the roms for anyway) edit the xml file as per above instructions (Use a free xml editor so you can view them in in any order -nice and pretty-, or just use notepad and search for the game) and edit it , save it then refresh MaLa list.
</game>
<game name="nss_smw" sourcefile="nss.c" romof="nss">
<description>Super Mario World (Nintendo Super System)</description>
<year>1991</year>
<manufacturer>Nintendo</manufacturer>
<rom name="spc700.rom" merge="spc700.rom" size="64" crc="44bb3a40" sha1="97e352553e94242ae823547cd853eecda55c20f0" region="user5" offset="0"/>
<rom name="dsp1data.bin" merge="dsp1data.bin" size="2048" crc="4b02d66d" sha1="1534f4403d2a0f68ba6e35186fe7595d33de34b1" region="user6" offset="0"/>
<rom name="nss-c.dat" merge="nss-c.dat" size="32768" crc="a8e202b3" sha1="b7afcfe4f5cf15df53452dc04be81929ced1efb2" region="cpu3" offset="0"/>
<rom name="nss-ic14.02" merge="nss-ic14.02" size="32768" crc="e06cb58f" sha1="62f507e91a2797919a78d627af53f029c7d81477" region="cpu3" offset="0"/>
<rom name="nss-mw-0_prg.ic1" size="1048576" crc="971ff812" sha1="4e8f5f0d160ca3e14db3d88c00f2e7e2dce4dca6" region="user3" dispose="yes" offset="0"/>
<rom name="mw.ic3" size="32768" crc="f2c5466e" sha1="e116f01342fcf359498ed8750741c139093b1fb2" region="user4" offset="0"/>
<chip type="cpu" name="G65C816" clock="3580000"/>
<chip type="cpu" name="SPC700" clock="1024000"/>
<chip type="audio" name="Custom"/>
<display type="raster" rotate="0" width="512" height="480" refresh="60.000000" />
<sound channels="2"/>
<input players="4" buttons="6">
<control type="joy8way"/>
</input>
<dipswitch name="Enforce 32 sprites/line">
<dipvalue name="No"/>
<dipvalue name="Yes" default="yes"/>
</dipswitch>
<dipswitch name="Difficulty">
<dipvalue name="Normal" default="yes"/>
<dipvalue name="Hard"/>
</dipswitch>
<dipswitch name="Lives">
<dipvalue name="2" default="yes"/>
<dipvalue name="3"/>
<dipvalue name="4"/>
<dipvalue name="5"/>
</dipswitch>
<dipswitch name="Time limit per level?">
<dipvalue name="104 sec."/>
<dipvalue name="112 sec."/>
<dipvalue name="120 sec." default="yes"/>
<dipvalue name="? sec."/>
</dipswitch>
<dipswitch name="Unknown">
<dipvalue name="Off" default="yes"/>
<dipvalue name="On"/>
</dipswitch>
<dipswitch name="Unknown">
<dipvalue name="Off" default="yes"/>
<dipvalue name="On"/>
</dipswitch>
<driver status="preliminary" emulation="preliminary" color="good" sound="imperfect" graphic="good" savestate="unsupported" palettesize="32768"/>
</game>