Download the enc_mp3.zip from here:
http://www.musicex.com/mediajukebox/mj_data/plugins/enc_mp3.zip zipped to the plugins folder in MJ directory, now make this two registry strings
for Normal MP3:
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\JRiver\Music Exchange\1.0\Media Jukebox\Plugins\Encoders\MP3 Encoder]
"CLSID"="{3FC1631E-50A1-4480-9D5D-A919484A0DF2}"
"IID"="{F48C2E06-C782-4C20-9447-1C2925F0F57D}"
"BufferInputIID"="{1E00D700-8DB7-4D78-BF6B-A217302C8522}"
"EIID"="{5AAF23C3-BEF5-4F2B-A5CD-EE75E2D37EA9}"
"IVersion"=dword:00000003
"Company"="JRiver, Inc."
"Version"="2.0.47"
"URL"="
http://www.mp3dev.org/mp3/"
"Copyright"="Copyright (c) 2000-2002, JRiver, Inc."
"Ext"="mp3"
"Configurable"=dword:00000001
"SelectedQuality"="128"
"Qualities"="32,40,48,56,64,80,96,112,128,160,192,224,256,320,Custom"
and for MP3VBR:
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\JRiver\Music Exchange\1.0\Media Jukebox\Plugins\Encoders\MP3 Encoder VBR]
"CLSID"="{3FC1631E-50A1-4480-9D5D-A919484A0DF2}"
"IID"="{F48C2E06-C782-4C20-9447-1C2925F0F57D}"
"BufferInputIID"="{1E00D700-8DB7-4D78-BF6B-A217302C8522}"
"EIID"="{5AAF23C3-BEF5-4F2B-A5CD-EE75E2D37EA9}"
"IVersion"=dword:00000003
"Company"="JRiver, Inc."
"Version"="2.0.47"
"URL"="
http://www.mp3dev.org/mp3/"
"Copyright"="Copyright (c) 2000-2002, JRiver, Inc."
"Ext"="mp3"
"Configurable"=dword:00000001
"SelectedQuality"="Normal"
"Qualities"="Low,Normal/Low,Normal,Normal/High,High,Custom"
Done