A couple of small corrections to that...
"Mining", as in (d)(d), is default after just the (d), unless some condition (I think that condition being "the cursor is not underground", but ICBW) makes the default "Tree-felling", aka that which is explicitly (d)(t) if you wish to force that, instead of mining, or switch back to it after some (d)(p)lant gathering or similar.
Secondly, if you have it correctly set to be a mining designation (or, indeed, any other one that applies to the square under the current cursor position) <enter><enter> will make a one-tile assignment at that spot if applicable. Essentially that's <enter><no-cursoring><enter> for a 1x1(x1) total designated tiles, where <enter><cursoring><enter> sets two diagonally-opposite corners of a rectangle. (Or, now that we have cross-Z designations, two doubly-diagonally-opposite corners of a cuboid.)
I still have the horrible feeling that OP is trying to mine on open ground (where (d)esignating a (c)hannel would be successful), although I must still take the comment "I know how to mine" at face-value. But that leaves me still wondering which particular "nothing" it is that is happening. It could be anything from not getting mining designations to appear on the map (because of not designating solid rock, but above-ground/on ramps/whatever) through to just not getting the (successful) marked designations fulfilled, as others read the problem as being, perhaps because there's no stairs or ramps or drift-mine-into-the-side-of-the-hill access into the area. Or (perhaps most exciting of all!) some strange bug where the (d)(d)<cursor to somewhere that you can dig><enter><cursor somewhere else><enter> combination isn't actually 'painting' the scene. Or a variant on that issue.
The bit where he says "{I} pick a point" might involve mouse-clicking, except that (where digging is viable) that's an even surer way of getting a digging designation set up, at least with default DF, although I think you can mess about with the INITs and stop that happening, or something. (ISTR, BICBW.)
OP: Please, is it working now? Is any of our advice helping? Are we just confusing you with so much information about every single possible problem that you might be having except the one you actually have? I'm just tingling with expectation about your reply, which probably means I need to go and lie down in a dark room, or something, but there you go...