I know...old post (and I wrote a lot, in hopes a reader cares why), but "greyed out install and remove buttons" / no reviews / no screen caps wasn't just driving me nuts; it's been annoying a lot of people. There are numerous irrelevant answers and altruistic guesses out there, even advice to give up on Ubuntu Software Center (USC) despite comments that Synaptic can be a good way to screw up your system...because the real issue is that you need USC not to care about the network state (especially when it's clearly wrong).
The most common fix appears to be Patches/Update USC/etc, and monitoring for errors by starting USC from a terminal (I had none), but for me and many others it looks like it's Network Connections. It didn't help that I answered the installer's "which interface is your primary" question thinking it was temporary, which turned out to be permanent (forcing me to edit /etc/network/interfaces), but I installed "Wicd network manager" because Network Connections was broken anyway. After this, USC always thought I was offline (so? I should be able to remove stuff, or install downloads). While you can always use the "File" menu in USC, it's counterintuitive for it to work when the same buttons are all disabled.
Skip the following paragraph if you never got a network connection of any sort.
With one, you can do a simple sanity check: If you are using another network manager (like Wicd or have edited /etc/network/interfaces) see if Network Connections needs a kick. Right-click its panel icon (an "up"/"down" arrow side-by-side. Google for how to put it there if it's missing). With USC open, try choosing "auto" for whatever interface is working just fine, and viola!--USC immediately enables the "install" and "remove" buttons. If this breaks your connection, you can use "dhclient", restart networking, use the menus, or maybe just restart (I'm not writing a network setup answer, so restarting should put you back where you were).
Regardless of online or offline, once I knew it was Network Connections, I found a post from Doctoa that says to run this in a terminal:
sudo stop network-manager
From http://askubuntu.com/users/68386/doctoa at
UBUNTU's Network Connection Manger can't detect Huawei ETS2051 Modem device!
In case this isn't a fix, remember that the "File" menu in USC still works despite what Network Connections thinks.