Apparently your missing the function for the twitter part.
Open your sidebar.php and comment out aktt_sidebar_tweets()
LIKE
//aktt_sidebar_tweets()

That may not fix the first error. But the first if not fixed may take a bit more to debug.
You may want to open the index.php and look to be sure that the last plug-in didn't add something to it. But I doubt that is the issue.

So do the second error first.

Easy solution is to use a different theme... Sounds like there's enough wrong with the one you're using that it's time to move on.

That is completely untrue.
Those are all PHP script errors.

hey try different theme or start from scratch, i just love elegant themes
-marylyn

What is your version of WordPress and what version of PHP are you using? Are you able to login to the WordPress admin section and double check everything is up to date?

I am not sure about the PHP being up to date, but I update wordpress every time it suggests it. I think WP is 3.0.3 now.
And c'mon, I had to search through hundreds of themes to get this one! its by site5. I didnt have money to purchase a better theme. Plus this theme has been up for over a month now without any problems!
find it here: contenttap.com
See how good is it?
UPDATE: PHP is 5.2.12

I reinstalled my wordpress theme, and wordpress, but neither worked. I think its something to do with my theme but I don't know what.

UPDATE: its not just my current theme, all the themes I try to use have wierd problems with them, like buttons pushed over, images intersecting each other.

This may seem a little basic but make sure the plugins you are using are compatible with the newest version of WordPress. Also, have you deactivated your plugins to see if it is a plugin compatibility issue between plugins. If you deactivate all plugins and the errors go away it's a plugin. Try activating the new membership plugin only and see if any errors show up. If there aren't any errors try activating any other plugins one at a time. Each time checking to see if your errors are back. Don't forget to refresh the page each time to look for errors. Hold down the Control key and press the F5 key to reload the page, not the browser refresh button. This will tell you if it is another plugin in conflict with the membership plugin. If this is a plugin conflict you can find which one if it is and you then have to choose which plugin is most important and is there another plugin that you can find to replace one of the 2 plugins in conflict.