To use one of the TI-BASIC variables, you need to first look for it with GetCalc, and then get the data from the pointer it returns.
Here's some code that will display the value of N if it exists, or a message if it doesn't:
GetCalc("varN")→P
If P
Disp "The value of N:",i,float{P}>Dec,i
Else
Disp "Variable N does not exist!",i
End
The var token is 2nd+[u] (the key 7). float{ is somewhere in the MATH menu, don't remember where, but what it does is it takes a nine-byte floating-point variable (the format the real vars are stored in) and converts it to a two-byte integer you can use in Axe.