In order to use classes and methods from another jar
1) You need to include them in the classpath. In essence you add them as libraries/references.
See: Adding Classes to the JAR File's Classpath
2) The method in question needs to have the appropriate modifiers related to visibility, e.g. to not be protected or private.
See: Controlling Access to Members of a Class
--A)If the method is protected: your own jar/.class must have the same package declaration.
--B)If the method is static
you can directly call it.
3) If the method is not static
you need to create a new Object of the class the method is declared in, and call it.
See: Java: when to use static methods
Object whatever = new MyClass();
whatever.getOffer(paramsGoHere);
See also:
Update
how to include libraries in java without using an IDE
Adding External JARs to Eclipse Buildpath
Adding Classes to the JAR File's Classpath (via command line)