um im pretty sure u need to call your carrier and update your package when i got the phone was not able to use msn/facebook tell i did but will up your bill...
now if u got msn and facebook installed and can use them but yahoo wont let u sign in
go to the options on the blackberry then advanced options then applications look for Yahoo! Messenger make sure its highlighted and hit the button with the blackberry icon "its beside the trackball" go to edit default permissions for the
-connections
usb - allow
bluetooth - allow
phone - prompt
location (GPS) - prompt
company network - prompt
carrier internet - prompt -interactions
interprocess communication - allow
device settings modification - allow
media access - allow
module management - allow
theme data injection - allow
keystroke injection - deny
browser filtering - deny
screen capturing - deny
security time reset - deny -user data
email/messaging - allow
pim - allow
files - allow
key store - allow
key store medium security - allow
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make sure u save the changes and see if it will go if not then like i said u will need to update your pack