Files on new computer are locked after using Migration Assistant and Time Machine
Sounds like the permissions are "wrong" or rather the owner is. Click on the folder, press Command+i, in the "Sharing &amp; Permissions:" section (on the bottom) and check if it shows your new user-account on the MBP as owner (first icon) and if the privileges are "Read &amp; Write".
Probably it will show a different user as owner. You could then try to fix the permissions by running Disk Utility, selecting you OS X partition on the left and click "Repair Disk Permissions" - but maybe this won't fix it, then you have to do it using the Terminal.
Maybe someone else can elaborate on that.

Edit 2011-12-18: Ok, I'll try myself:  

Something I didn't know yet but found now looking for an easy fix:  

From Troubleshooting permissions issues in Mac OS X  

Note: In Mac OS X 10.5 and later, while started up ("booted") from the Mac OS X 10.5 installation disc, a user's home directory permissions can be reset using the Reset Password utility.


But in Lion they removed the option form the menu, so if you boot (holding "alt ⌥") into Recovery HD


Wait for “Mac OS X Utilities” menu to appear, indicating that you are booted into recovery mode
Click on the “Utilities” menu and select "Terminal"
Type the following: resetpassword
Confirm the user account and then the password change and reboot Mac OS X 10.7 as usual with your new password  



Copied from Reset a Mac OS X 10.7 Lion Password

I didn't try it myself, but hopefully this should reset/repair your home directory permissions.



Maybe even better solution using Finder in normal OS X:


In Finder select the folder
Press Command+i
Expand "Sharing &amp; Permissions" section (Unlock if necessary 1)
If you're not in the list of owners, press [+] 2
In the upcoming list, select your new user
Finally click the gear 3, make your new user the owner, apply to enclosed items
 


and delete the old user by selecting his name from the list and clicking [-]







Easiest way, if you're not afraid of the terminal:
Assuming your new user is named harmen  

sudo chown -R harmen /Users/harmen  


This will make you the owner of everything inside /Users/harmen
Read more here  

Good luck.