Sorry about the late response!
There are two things that could cause this.
1) The "Instant Web Publishing acccess to this file" option is set to No users. To resolve this, open your database and go to File -> Sharing -> Instant Web Publishing. From there, make sure your file is selected in the lower left hand corner and select "All Users" to the right of it.
2) The other possibilty is that you don't have an account with the fmiwp extended privilege enabled. To access an IWP database, at least one account needs to have this enabled. To do this, head into File -> Manage -> Accounts and Privileges (Security for FMP 11). From there, select the Extended Privilege tab, choose fmiwp and then click Edit. You should be presented with a dialog that will allow you to enable this extended privilege for whichever accounts you'd like to access with.