ATI Eyefinity Technology can support up to 3 displays using a single enabled ATI Radeon™ graphics card – the number of displays may vary by board design and you should confirm exact specifications with the applicable manufacturer before purchase. ATI Eyefinity technology works with games that support non-standard aspect ratios, which is required for panning across multiple displays. To enable more than two displays, additional panels with native DisplayPort™ connectors, and/or certified DisplayPort™ adapters to convert your monitor’s native input to your cards DisplayPort™ or Mini-DisplayPort™ connector(s), are required. Digital management rights restrictions may apply.
While your HD 5450 has a Displayport, the implementation of it does not seem to meet Eyefinity specifications.
Have you tried installing the most recent drivers.