how to set the probabilty by useing neighbors variables

If it is a wired network, then you may easily resolve neighbor list.

But in MANET, resolving neighbor list is not that much simple.

You can prepare a neighbor list by sending one hop hello broadcast and receiving reply messages of neighbor nodes.

You can directly derive the neighbors from global ns environment using “god”. You may refer the following link for how to use “god” for deriving neighbor list in MANET. But it will also require good knowledge about compiling ns2.

http://www.cse.msu.edu/~wangbo1/ns2/nshowto2.html