won't let me add a widget

Try
?deactivating all plugins (yes, all) to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s).
?switching to the Twenty Eleven theme to rule out any theme-specific problems.

You also need to update WordPress to the current version.

have tried deactivating plugins and did not help. am already using twenty eleven. the guy who built my website says i don’t need to update to current version because the update doesn’t change much, he said to wait for bigger update. i have asked him a hundred times to help me fix this but all i get is lip service, don’t dare update on my own in case something happens and i don’t know how to fix. anything else i could try?

You are getting lip-service.

There are significant changes between WP v3.4.2 and the recent v3.5.1

But as kmessinger pointed out, you need to deactivate all plugins and use a default theme to narrow down the offender.
If your using the WP twenty eleven and you still have an issue, you should switch to another clean WP default theme like twenty ten.
If no other default themes exist in your install, then re-download one in your admin area… “appearance–>themes” menu.

the guy who built my website says i don’t need to update to current version because the update doesn’t change much, he said to wait for bigger update.

This guy doesn’t know what he is talking about. Time to replace him.
If you can’t do it yourself, try https://jobs.wordpress.net/