You would need to use 'formula is' from the first drop down list and set the formula to:
=MOD(A1,7)=0
Then design your conditional format as normal.
Note, A1 should refer to the top left cell in the block you're adding the conditional format to, and you should select the block beginning with this cell, otherwise you may have problems with referencing.
If it's only one cell, then of course the address should be the cell's address.
You could use this form:
=MOD(INDIRECT(CELL("address")),7)=0
but that seems a bit long winded.
edit: VoG's solution is better if you need to ignore blanks, but in terms of cell referencing, mine's worth a look too. You could probably adapt them to get the best of both worlds.