I assume the question is more specific about how to load a package when you edit notebook files on the development environment of Wolfram Workbench. If you installed Wolfram Workbench in Windows you should not have a problem.
But if you managed to install Workbench 2.0.138 on Linux, you may find out that after importing your application projects or even starting a new application project in the usual way with
<<YourApp` it fails to load your package.
The reason has been fully explained by @Leonid Shifrin above but solution has not been exemplified. It is a problem of appending the application project, i.e. package path, to your existing $Path variable. Somehow Eclipse-Wolfram Workbench does not add that automatically. You may try to right click with the mouse on the project and check the properties. I have tried to configure e.g. Mathematica/Project Paths option without any luck.
Therefore I added the project application path programmatically using the following command on a testing notebook file:
AppendTo[$Path, FileNameJoin[{$HomeDirectory, "WolframWorkspaces//Base//YourApp"}]] This way it works.....
PS: Note that the application project base of your Wolfram Workbench environment should have been set at
FileNameJoin[{$HomeDirectory, "WolframWorkspaces//Base"} Use Wolfram Workbench menu File->Switch Workspace->Other...
