If you have > 51% hashing power for a long period of time then you could theoretically choose an arbitrary block some time in the past (the further back you go the more difficult and time consuming it will be) and begin mining from that block. Including (or making up) whatever transactions you want as you go. Eventually you will have a longer chain than the "main" block and unless manual intervention is made you will orphan all of the other blocks and be able to rewrite history.
Dylan16807 is correct. You can't forge transactions. You can create an alternative history, but each transaction still requires holding the necessary private key to sign the transaction. If you're trying to make up new transactions out of whole cloth, you can only do so if they're transactions that you could have otherwise legitimately created.
What the 51% attack lets you do is remove transactions, which lets you double-spend. And of course there may be other benefits too. For example, if someone is using the blockchain for purposes other than sending money to people (e.g. notarizing a document, proxy transactions that represent movement of physical goods, etc) then removing transactions may be beneficial without double-spending.
