Ask yourself, "what does it mean to be in the lower right triangle" in terms of i and j? I.e., what test could you make on i and j to determine if you were in the lower right triangle? Then code that up inside your for loops. E.g.,
mysum = 0;
[row,col] = size(v);
for i= 1:row
if( in lower right triangle )
mysum = mysum + v(i,j);
So, you just need to figure out a test you can make on i and j to determine if you are in the lower right triangle, and insert that code inside the parentheses as noted above. If you can't figure this out immediately, write down all of the i,j pairs for a small matrix on a piece of paper and then look for a pattern that you can exploit and code up that isolates the lower right triangle. Do an example for both a square and non-square matrix to make sure the code you come up with works in both cases.
SIDE NOTE: There are ways to do this without a double for loop, but you should first learn how to do this with for loops.