what are possible isomers of (meso-stilbenediamine)(isobutylenediamine) palladium (II)?
The use of the term "meso" in the compound name indicates that both phenyl groups are on the same side of the molecule, "cis" to one another, not "trans".

Here are the tetrahedral and square planar representations. 



Note: I used $\ce{Pt}$ instead of $\ce{Pd}$ as the central metal, but it makes no difference in terms of isomers.

The tetrahedral molecule has a plane of symmetry containing the "top" 5-membered ring, bisecting the 2 methyl groups on that ring and then bisecting the "bottom" 5-membered ring with the 2 "meso" phenyl groups. Since the tetrahedral molecule contains a plane of symmetry it is achiral and has no isomers.

In the square planar form, there is no symmetry element.  Therefore the square planar molecule is chiral and exists as an enantiomeric pair of isomers.