Mail app 'use this mailbox for Junk' not working for IMAP Spam folder

Hi there. Just setting up my new Intel iMac (now that my old PowerPC iMac is getting tired).

I use webmail from 1and1 and have sucessfully managed five accounts via Mac OS X MAIL and its IMAP configuration for the past six years.

I have managed to setup the five email accounts in MAIL on the new iMac (MAC OS X Lion) but am now struggling with two things:-

1) Select my IMAP 'spam' folder, go to MAILBOX / USE THIS MAILBOX FOR / JUNK. For some reason the folder (and content) is not moving to the MAIL 'Junk' folder for the account

2) I cannot create a new folder within the INBOX of an account. I select account in left-hand pane, then MAILBOX / NEW MAILBOX / enter NAME - however I've noticed that the 'folder' is being created at the top-level of the email account, rather than within the INBOX. This was always possible to do within MAIL on the old iMac (running MAC OS X 10.4). My short-term workaround is to log into webmail, create a new folder within the INBOX structure, then refresh iMac MAIL which then shows the new folder within my INBOX.

Appreciate your help. Apologies is this is a tad long-winded. I'm a newbie!

Many thanks