Assuming that you are developing a WebApplication and not an AIR desktop application, you are allowed to access either network files or local files, but not the both of them. When you run from FlashBuilder, your program is executing from a SandBox and such restrictions won't apply, how ever when you deploy outside, you will get a security error.
If your application does not need to use any network features, you can specify use-network=false as part of the compiler options which will allow you to access local files.
To add/change Compiler options, Right click on your project in Flash Builder, select Properties, then Flex Compiler, and enter the option in Additional Compiler arguments. (I am using Flex Builder 3, so it might be different on the Flash Builder that you are using)
Flex Compiler Options