I downloaded an application which is a free application to my sd card but how should I install this in my phone?
Just use a file explorer (Astro or OI) to navigate to it and select it. Note - it has to be a .apk and NOT .zip if it is .zip rename it to .apk.
You are right to be wary. There is much bullcrap. Be wary of me too, because I may be wrong. Make up your own mind after you evaluate all the evidence and the logic. - Mark Rippetoe
In Astro, you should see a folder called sdcard. Usually the apps you download go into the /sdcard/download folder. If the file is there and has a .apk extension, just click on it. If it's a zip extension, you will have to rename to <whatever>.apk
If the extension isn't shown, long press the file and go to details. Also make sure (if this is a 3rd party app or non-market app), you have to go to Settings/Application settings and enable Unknown sources at your own risk.
I added it all and then went to add DroidRootHelper (stupid app i know, but i was using it to show a friend something simple) and Astro won't let me install the .apk. I allowed unknown apps and such in the settings.
Here's a screenshot of the icon next to the DRH apk i want to install.
This issue came up a long time ago, but i just forgot how to fix it.