Iam sorry !Sounds too funny!
Are you able to open sbsettings (swipe the status bar on lockscreen left or right) ? If yes you can select the safe mode (orange button) to get into safe mode so that the addon get's
disabled and you are again able to unlock your device.Of yourse you need to delete the app via cydia or the way i pointed out in my last suggestion *
Not working ? OK !The hard way this time .
Reboot the phone while keeping the volume up button pressed during the phone boot process ,by doing this all mobile substrate addon's should be disabled and the slider should be back .
If this also fails you can hook it up to your pc or mac download iTools One-stop manager of your iPhone, iPad & iPod Touch
and delete the lock plist and dylib in /Library/MobileSubstrate/DynamicLibraries .
One of these solutions will work.