Normally, if these are entered in the first year you've set up in Corporation Tax, there will be a
brought forward
option in the Event column of the
Asset Transactions
dialog.
If the brought forward option is not available, it's because you're entering the information in a subsequent year. You can go back to the first year and enter the information there.
If you have a pool with a nil value brought forward, you'll need to enter the nil value row before you can dispose of any items.