You can upload data as a new subject or as a new session under an existing subject. In the new session case, you have to pay attention to be sure that the subject ID is exactly the same, as the subject under which you would attach the new session. To do so, you have to use the 'Upload session data' or the 'Upload your data' button.
To avoid the trouble about the subject ID, there is a more direct way to upload a new session.
Go to 'My data' right click on that subject ID under which you would like to upload a new session. Then, click on 'Upload new session'.
By default, the upload will be given a session ID with an automatically generated number to be able to differentiate the sessions. To create your own ID, description, name and define the type of the uploaded file (Medical image data or MRI) click on the 'Show advanced options' text.
To upload the new session data, click on the 'Choose ZIP to upload' button. After the upload, if you want you can specify the advanced options (type, name, description and subject ID) and click on the 'upload' button to finish the session uploading.