What URL should I go to in order to get to the login page?
Like Kristopher says it may be something else.
However for jetty check that /openmrs-core/webapp/target/modules is the location of the modules folder /openmrs-core/webapp/openmrs-runtime.properties is the location of the runtime-properties file that is how my jetty is setup
Also initial setup creates runtime-properties files so it must be somewhere.
I am also having this problem, after successfully building the latest changes of openmrs-core and legacyui, i copied the .omod file into ${HOME}./OpenMRS/modules folder, and tried to access localhost:8080/openmrs/admin/index.htm and i get a 404 error.
That modules folder location is used by tomcat. If running jetty you have to move the modules folder and runtime properties to the locations as per my earlier response.
So I moved the modules folder and runtime-properties file to the locations you specified but it still is giving me the HTTP error 404, please advise
Make sure you have pulled the latest platform and module changes
everything is up to date according to git and its still giving the same errors
Everything works fine now on my end. 
Jie now you have the necessary information to setup OpenMRS 2.0 platform and the legacyui module, I will suggest you delete the source folders for core and legacyui, download fresh copies and start afresh. This will ensure that you have the latest versions from github. This thread contains all the information necessary to successfully run the legacyui module against the platform.