Can you please give more details?
I assume you guys has already jailbroken your device.
I prefer using installous to install ipa file, not itunes. It never disappointed me.
if you have not yet got installous, go to cydia and install it, add http://cydia.hackulo.us/ as source and you'll find it.
Then, transfer the ipa file to installous download folder, using SSH or USB, any method of your choice. I recommend using iFunbox, easy and quick.
the default location of installous download folder is //User/Documents/Installous/Downloads/
Then you can see the file when install the ipa without any difficulty using installous. |