Yesterday I visited FindMyPast again and found the entry you mention here. John Campbell No: 451548 (Page 232)
As I have already said,
BT 114 is the index only. The Seaman's Ticket is in
BT 113/226. The ticket can be seen on FindMyPast.
From the menu choose
Search, then
All Record Sets.
Choose the record set
Britain, Merchant Seamen 1835-1857.
In the First Name box, enter
John.
In the Last Name box, enter
Campbell.
In the Series box, enter
Bt113 (no space,
allow it to autofill).
In the Keyword box, enter
Dundee.
Don't enter anything else.
Click
Search.
There is only 1 result.
The above method should enable you to see the ticket for John Campbell No: 451548 in
BT 113/226. It is on
folio 194. It is
image 390 of 516, in case that helps.
Whether this is your man is, of course, another question.