how can enter the opening balance for VAT

If you want to update the VAT Entries without changing your General Ledger do the next:
Go to G/L Journal, create a line for each VAT type : for instance 1 line for Purchase, National, VAT nn%, another line for Sales National, VAT nn% etc...
On each line the G/L account is the corresponding VAT Account, the VAT Bus. Posting Group and VAT prod. Posting group are the corresponding (for instance "National", "VATnn") and the balancing Account is the same account no.
Before to post change the VAT % of each VAT Posting Setup Line to set it at 100%, post, then change again the VAT posting setup with the correct %.
As amount simply use the double of each VAT open entry you want to have (for instance if my purchase, National, VATnn open amount is 100, use 200).
Note: this "open entry" will allow you to have a normal VAT declaration.

Refer to NAV 4.0 SP3 HF1 & HF2 feature instruction manuals. As far as I remember there is a specific manual for this purpose.

Download HF1 and HF2 from the partner source/customer source.