https://en.numista.com/forum/topic3546.html
That topic may also help.
But yes, you need doubles first to add to Exchange (sixth column, after the grades). So you now know how to add coins at least, which is good.
P.S. I also see you've added another coin
Collection 53 coins / 7 countries (view)
and the link is now showing. Thanks!
Also, once you have some coins in your exchange list, perhaps for your first swap you may want to exchange with a member in your own country, which would make postage cheaper. Go on left to Exchanges - Start a swap, then to Swap partners - Find members near your home!
Also there is the cross or heart markings for each coin - if you do not wish to get a coin it has a cross, or a heart if you do want one. Takes a while to mark all coins that way, but gets easier with time or number of swaps. Every moment I get I like to mark some more coins! An example is below, where I clicked on a member's name, then 'Coins to exchange' ...
The first column to the left of the date is the symbols showing
for me the coins I have entered already >
Green dot: You don't own this coin
Blue exclamation mark: You own this coin, but not this year
Grey tick: You own this year
Now that your collection is viewable, I see, for example, you have 2007 of this 5 Pesos coin ...
https://en.numista.com/catalogue/pieces589.html
so if a member you want to exchange with also has that coin and year for exchange, you would see a grey tick, if they have a 2005 of that coin you would see a blue exclamation mark, of if they have say a 10 Pesos of 1986 then you would see a green dot (as your only Mexico coin is a 5 Pesos and the other member has a 10 Pesos). And so on ... !
That column you don't have to mark anything specifically, as the system takes note of all the coins you have already marked. On that image above is a second column of symbols to the left of the other one. That is for where you have been to a coin page and marked either 'I wish to get this coin' which would put a red heart at the top of that coin page, or marking 'I don't wish to get this coin' would put a grey cross. So in my image I should click on that last Bahrain coin to see if I would want it; then every swap in future would always show whatever I chose - and would not be displayed if I clicked on 'Hide X coins'.
All a bit complicated perhaps, for a beginner, but it shows what can be done.