No clue on macs so forgive the lack of specifics. Do you have a numpad? 7,9,1,3 work for me. You can also click_hold and drag to the spot you want.

I run this game on my Mac. Use the mouse to move units diagonally or in any direction: Click on the unit and drag the mouse in the direction you want to move. Works for me.

Hello,
I was struggling with the same problem. It seems that you play it on a laptop with no numerical keys? During game-play you can press the command key and a menu-bar for dosbox appears. In one of the menu's, I'm not on my mac right now, you can emulate numerical keys. This way certain keys on your alfa-numerical keyboard now functions as numerical keys. This way you can move diagonally.

I find that if you just hold down the mouse button over the square you want the unit to move to, after a second the cursor changes and says "to". If you then release the mouse button, the active unit will start moving toward the destination. This works better than the click and drag technique because you can select a destination that isn't on the same screen.

I realise this thread is 6 years old at this point, but i have some insight on how to solve the issue that might still help OP or others in the future solving this problem.

Solution:

When Dosbox is running (in this cas Colonizattion) you can upen up a keymapper by pressing CTRL + F1, in there you can map num keys to something else. In my case i deleted the PrtSc, ScrLk, Pause, Ins, Home, PgUp, PgDn, Del and End, i then added/mapped the numpad keys to these instead as these conveniently form a square cluster on my keyboard, making them a great replacement for the numpad keys.

When you are done just click save and then exit. You now have a custom keymapping for Colonization!

Reference: https://www.dosbox.com/wiki/Mapper