Okay, have a look at this link -
http://help.adobe.com/en_US/flex/using/WS2db454920e96a9e51e63e3d11c0bf69084-7f3c.html
In general if you need to use properties file, it would be better to create bundles. From the link you can see that the below are the steps to accomplish our task.
1) Create property files.
2) Include the names of the created property files under the bundle list.
3) Then include the bundle list in the Flex compiler arguments.
4) Then use them from Resource manager.
In case you want a conventional Property reader sort of functionality, you could use the url class loader and give full path.