You don't access Mobile Substrate. When it kicks in and puts the iPad into "Safe Mode,", it's because of an automatic
feature built in. It either protects the iPad ... or it doesn't. You don't have control over it.
Essentially, it's saving you from the iPad's reaction to some tweak you've put on it. It's kind of like a "time out" for the iPad - while you sort out the situation. And, not all apps use Mobile Substrate, nor will the failure of just any ole app cause it to kick in.
If you do something that cause Mobile Substrate to put your iPad in Safe Mode, a restart will usually bring your iPad back to normal. If a restart doesn't fix it - Mobile Substrate will probably kick in again, which is your hint to start figuring out what tweak you added to the iPad caused it to start crashing.
Regardless, getting a reboot/respring into Safe Mode - which is caused my Mobile Substrate's auto feature - is cause for you to take a step back and try to figure out what you just did to the iPad to cause it to have a wee bit of a hissy fit. Just count the lucky stars Mobile Substrate protected you and gave you that chance to fix. Else - restore time!
I'm making it sound more complicated than it is, but the main thing you want to do, to be prepared, is have SBSettings installed so you can have a fighint chance to manually put the iPad into Safe Mode should Mobile Substrate
not auto start.
There, now that
that's clear as mud ... any questions?
Marilyn
P.S. Worse comes to worse, Google "mobilesubstrate." It's a very interesting feature ... one I'd take years to master any understanding of. I am always SO impressed with programmers ...!