{D} represents an additional cost, not a mana symbol. {D} can be paid for by giving up one potential land play you have remaining this turn. Boulder Jockey’s mana value is 3.

To determine how many land drops you have remaining this turn, compare the number of lands you can play (which is normally one but can be increased by other effects) with the number of lands you’ve played and land drops you’ve given up this turn. If the number of lands you can play is greater, you can play a land or give up a land drop. You can’t give up a land drop you don’t have.

You can never pay for {D} when it’s not your turn.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.