A well set-up project in your IDE makes development and debugging work much more productive.
If you have an existing Maven project, import it using File > Import… > Maven > Import Existing Maven Project and follow the wizard.
Dependending on your project structure, Eclipse may not recognise the source directories and Java packages are not displayed correclty in the Project Explorer. To teach Eclipse where to find Java files and packages, go to the project settings and change project nature to Java.
Then Right Click > Properties > Java build Path > Source Tab > Add Folder > Select src folder. This makes Eclipse aware of additional Java source directories and they should show up in the project.
If your project is messed up in Eclipse and you want to start the import from scratch you have to delete all of the following files in your project folder. (You can use Windows search to find them, then select all and hit Delete.)
.classpath .settings (files and directories) .project