adder in logisim using only 2-input-NAND gates
If I understand your question right, you want to build an incrementor.

First you need to build a halfadder. If you do not can or want to design it, you can ask logisim to do it.

choose project/analyse circuit. add two inputs (a,b) and two outputs (o,c).

then fill in the logic table for the half adder.

a b | o c
...............
0 0 | 0 0
0 1 | 1 0
1 0 | 1 0
1 1 | 0 1 

then push build circuit and check both options (2 input gates only and nand only)

logisim wil build the circuit:



then you can connect 8 halfadders to build a 8 bits incrementor.