If the Administration block is not showing, check to make sure the instructor hasn't removed it. If they did, you can re-add it by Turning editing on and then on the right side, find the Blocks block. Select the pulldown menu and select Administration. This will re-add the block.
If the Administration block is there and students can't see the Grades link, check to make sure the instructor hasn't it in the course settings. To check, select Settings in the Administration block and then locate the Show gradebook to students field. Make sure it is set to Yes and save your settings.
If after all this they still don't see it, then it may be a case of permissions that you have set for each role. Try the options above first though and then report your results.
Happy
oodling!