Dear Galil
The formula syntax "=ADGET ("gldata.Bal",$C53)" is good, so that cannot be your problem - however, you have not specifically said what did not work after you tried to edit the formula to change the reference cell - what happened exactly? After you replaced the old "$C53" with "$C58" - did you press Enter to accept the changes? Could you see $C58 in the cell formula after your changes, or did it still say $C53? Or - do you mean the balance was still not showing the actual balance after you successfully made the change? Did you try pressing F9 to recalculate the balance?
Another thought is: are there actual GL Account numbers in column C? If you click on cell C58 - is this where the new assets account number is? The cell should only contain the following syntax - e.g. '9999-` where 9999 represents your account number.
Again, you cannot grab accounts in the Chart of Accounts view, only in Account Group view.
Since you created a 'new' database for use with Adagio and have "imported certain files" from that database into the Adagio database, and are comparing this with your existing ACCPAC Plus data I would expect possible issues, as these two databases are not the same as far as the accounting structure, are they?
Another thing you could try that would maybe shine some light on this, would be to copy your existing ACCPAC Plus G/L data into another new directory, then point Adagio Ledger at it. Go into FR and import one of the DOS Balance Sheets. Look at how it is configured in relation to the 'new' database that you are struggling with and make the necessary changes.
It is becoming difficult for me to get a clear sense of what is happening here, Galil. My suggestion for contacting a Dealer in your area, or signing up for direct Technical Support stands. A Dealer would be able to help you right in your office while being able to work with your actual data that you have set up and train you in the process. I cannot do this from the Forum. To go any further with this I would need to have your data here to see what you are seeing and be able to advise you accordingly. I cannot do this if you are not on a Combined Support Plan. 
Sincerely
Softrak Support