You probably understand this, but read this whole thing...
Your hosting company supports many users like you on the machine your connecting to. To do that they will create a folder structure similar to your Mac which is /Users/username, where username on the hosting site is likely the first part of your domain.com name ,or perhaps another name you told them to use. The name your logging in with anyway.
When you connect, it is normal for ftp to connect at the top level of your directory structure, so really you shouldn't have a permission problem. This is because each user should be able to read/add/remove/alter their own files. Your username has unique ids that are associated with the files you create and own.
First you need to know if you are ending up in your directory. It would be odd not to. If your using the Terminal command line tool 'ftp' then the 'pwd' command tells you what directory your in. If your using a GUI tool then it should say that somewhere or via a button. You need to be in or below your top level (known as root is the biz) directory.
The ls command, an 'ftp' Terminal command, I gave you will list several important things. A GUI tool may not list those items, but might have an option to show the information, such as a "Get Info" or "Show Properties" button.
The important bits are, owner, read permission, and write pemission. There is also execute permission so your scripts will run, but I doubt that is important here.
An ftp ls command will display something like this;
-rw-r--r-- 1 xstep guest 317 Jan 22 2003 fdev.txt
The first character, a dash says this is a normal file. A d would say it is a directory/folder. The next three charactes, rw- tells me that the owner, 'xstep' of group 'guest' has no execute permission but does have read and write permission. So I can edit and delete this file. The next 'r--' says the group can read the file. The last 'r--' says the world can read the file, which of course needs to be true for you to serve the page. ie; The web server needs to be able to read the file to present it to the browser asking for it.
Finally we get to your problem, '"blank" cannot be modified'. The words 'cannot be modified' suggest your trying to put a file onto the server. Only two things would stop this. 1) Your trying to replace a file you don't have permission to write to. 2) The directory your writing into is not owned by you or you worse, you do not belong to a group that can modify the directory meaning your someplace you don't belong.
If the "blank" is litteral, then perhaps you are not actually connecting to the server. When you post, you should post the exact message.
One final note. Often the server company sets up a directory within your top level directory that they say is where your web files are to be placed. The common name is the past has been public_html so if you see that or they told you another name, enter that directory.
I also suggest you review any notes or FAQs your provider has.