Can't launch secondary computer or don't know how.

Sorry to hear you are having trouble.

First, you are going to want to make sure you have MP installed on all the machine you want to use MP with.

For Seamless connections, you are going to want to make sure you can see the monitors of the Primary and Secondary (on the same desk). If the Secondary is in another room \ location, where you cannot see the monitors, you are going to want to use KVM connections.

Next, you would want to make sure that the Secondary computer you are trying to reach is on the side of the Primary:

Example of Secondary monitor(s) on right or Primary monitor(s): http://sd.stardock.com/sean/JingImages/2014-11-18_1531.png

Diagonal placement of a Secondary will not work:

http://sd.stardock.com/sean/jingimages/2014-08-05_0950.png

But above and below is fine.

Here is further information on Seamless connections:

http://help.edgerunner.com/Multiplicity%203/Static/default.htm?kvm.htm

If the PCs are not on the same desk (other rooms \ locations), or they are close but one is headless (no monitor) you are going to want to create a KVM connection.

KVM connections will open a window on the Primary where you can view and interact with the Secondary computer. For more information on creating and using a KVM connection, please refer to the following:

http://help.edgerunner.com/Multiplicity%203/Static/default.htm?kvm.htm

i installed the app on the secondary computer... I get a message on primary that the secondary is connected..  .. so how do I control the secondary from my primary keyboard and mouse.?

If you have an understanding of the differences between seamless connections better now, you will need to have at least purchased Multiplicity KVM to do what you are asking (the trial is Multiplicity KVM Pro btw).

I would also encourage you to read the sections of the manual I pointed out - they go into much greater detail.

both computers are side by side both connected to the same monitor.   I was wanting to get rid of one keyboard and mouse.   I have no idea what you are talking about use KVM?  

For what you want to have, to be able to remove one PC from needing a monitor (make it headless), you are going to need Multiplicity KVM:

https://www.stardock.com/products/multiplicity/

https://www.screencast.com/t/LkNkehjH0sxz

it sounds like I spent money on the wrong thing.. once I got hooked the other shoe fell.   

You can upgrade here: https://www.stardock.com/products/multiplicity/