Im not sure why you would have been told this, I doubt anybody from FMI saying you could.
Perhaps they where talking about a volume licence or the 5 pack they offer.
If you bought the 5 pack, then yes this would be fine... but if you bought a single licence, that is exactly what it is.. for a single computer.
The exception here is that FileMaker Pro Advanced allows you to install on two machines, but still only use one copy at any one time,
in other words, this is ideal for deploying cross platform solutions, since you are allowed to install on both windows and mac.
But you still can not use it to share the database between the two (besides it being against their licence agreement, the software will check)
"We where informed on purchase of FMP 11 that we could run a single license on 5 separate computers,"
This would only be true if you used filemaker's Instant Web Publishing to publish the database. Then, up to 5 simultaneous users can access the database using their web browsers. This is not an approach for a beginner to attempt and there are significant limitations on how the filemaker interface will function in this environment, but it can be done.