You need to add the two lines:
export X3DNA='/home/sima/x3dna-v2.4'
export PATH='/home/sima/x3dna-v2.4/bin':$PATH
into your ~/.bashrc file so that you can access 3DNA commands each time you open a new terminal window.
Or, you can place the two lines into a file, and then source that file each time you open a new terminal window.
Xiang-Jun