Extension Manager 5.5 won't install my transitions with .XZP file. Why?

I have downloaded a set of Transitions from Filmimpact.net for Premiere Pro CS 5.5. It is a .XZP File

I Double clicked on it and Adobe Extension Manager CS 5.5 opened up.

I accepted the License agreement and it opend up.

I went to install These transitions and I get the following Message

"The Extension 'Transition Pack 1' does not contain valid signature. The extension will not be installed."

What does this mean and how can I fix it?

I want to install these Transitions in Premiere Pro CS 5.5?

Thanks in advance