How to step in the disassembly view?

I'm trying to debug something at the assembly level (mostly compiled from mix of C).  Once I get to a point in my C code I then jump to some memory address where other code is loaded.  With a breakpoint there I want to open up the disassembly view and step in that instead. But in that view F5 or F6 both say they cannot find the function (which is unsurprising as this is the startup code of a new program). 

Is there anyway to step at the assembly level?

Also, in a disassembly view sometimes entering a new hex location to disassemble just does nothing (source sync etc. are turned off)