The assembler project is described in detail in the handout (PDF). Notice that, even though this project is apparently very different from the compiler, it can share a very large majority of its code with the compiler. The main difference is really that expressions are evalated to produce integer constants, rather than translated into code; integer values can easily be passed back using the return values of the expression parsing routines.
Rather than an online version of the project, there is an online version of AIK specification for the project: http://aggregate.org/CS/aik.html. You can compare your assembler output to the output of AIK using this specification.