"looks like we should not delete any existing components in the domain model that is given but we can add some components ... "
Try to stick to Business Domain model as much as possible. If you want to deviate-you can but provide the justification for that.
"I'd like to know if we can depict the shopping cart and catalog components in the class diagram. The requirement and the usecase talks about these ..so i think unless we have these in class diagram we'll not be able to have required communication among objects to achieve the desire solution (which is an online retail system/site as specified in the requirement). "
Like everybody, when they started working on the assignment-they were puzzled in terms of:
1) How much detail to provide?
2) What goes in where?
Your class diagram should be build around the Business Domain model-how much detail you want to go depends upon you?
There will be a temptation of putting everything in the class diagram and make it too high level-you need to make a decision.
Read the assignment-what it says "Public method names referenced in other uml diagrams like sequence diagrams should be provided"
In fact you need to come back and forth between class diagram and sequence diagram few times and then make sure that they are in sync with each other and with overall solution.
Best way to judge your progress is to ask yourself-whether it makes sense - is your documented solution is making sense and solving the given problem from the Architect's prospective? And then get it reviewed by you peer(may be already
SCEA or non SCEA Architect)
Hope this helps.