There are potentially multiple issues that can cause this...I will try to give you the answers that I have found from personal experience
its confusing but there are 2 variations of Adobe Flash depending on the technology NPAPI or PPAPI- you need to ensure that you have installed the right one.
You need to install the PPAPI version and then ensure that it is working in the browser correctly in the machine...then try again.
The other area that you can also run in to is a mismatch between the client version and Flash version based on whether you are using the 32 bit or 64 bit SAP Business One client.
Make sure that your version of the Flash Player matches the client ie 32 bit B1 Client = 32 bit Flash Player
For those of you that care about such things here's a good explanation of the differences between PPAPI and NPAPI