How do I remove an app installed if the buttons in "Manage Applications" are locked and can't be pressed? I didn't download the application from Google Play and got the apk from a website and installed it on my tablet. But the app wasn't the one I needed and it was just making offers for me to get other apps.

Now I want to uninstall it but in "Manage Applications " all the buttons there are locked and can't be pressed, including the uninstall and move to SD card. 

What do I do to get rid of it? It's taking up space on my internal memory.
Some apps register with the "Device Administrator" to avoid accidental/unwanted de-installation (e.g. child-protecting apps, so the child does not simply uninstall it -- also stuff which implements additional "rules", such as MS Exchange related apps). In this case, you need to remove them from the "Device Administrator" first:

Go to Settings&rarr;Security&rarr;Device Administrator and check if the app is listed there. If so, remove it from here. Now you should be able to uninstall the app.