Finding documents shared with you in Google Docs can seem difficult at times. Hopefully this brief walk through will shed some light on the situation.
Navigate to Google Drive by either selecting it from the App Menu (Grid of 9 squares in upper right corner of your email) or by typing drive.google.com in your web browser.
OR
Once in Google Drive you will see a menu on the left. It lists: My Drive, Shared with me, Recent, Google Photos, Starred and Trash. My Drive and Incoming are the important menu items in this case.
Click Shared with me to get a list of all Google Docs, Sheets, Folders, etc. that have been shared with you by another Google user. You can see who the owner is to the right
of the file (if files are listed as icons you will need to click the 
on the upper right to get a List view giving you additional details.)
The documents are available to you but will only be able to be found in the Shared with me folder. If there is a file you would like to have readily available to you there is an option to move it to My Drive. To move it to My Drive you can right click on the file and click Add to My Drive (you can, also, open the file and use the File pull-down menu.)
If you click the Remove option you will simply be removed as someone the file is shared with. The file will still belong to the owner and you need the owner to re-share with you if you decide you want to view the file again.
When you click the Add to My Drive button a popup will appear that will give you an option to move it to an existing folder, or to create a new folder, in My Drive. Click the x if you'd just like to reside in the root of My Drive. If you would like this file to reside in a folder in My Drive click Organize.
If you have an existing folder you would like the file to reside in click the name of the folder and then Move Here.
Click the 
if you would like to create a new folder for the file to reside in.